1. A scab is a dried plug, mostly blood, that covers the wound
When skin is cut or scraped, bleeding starts, and within the first few minutes tiny parts of the blood called platelets stick to the injured spot and begin to clump together [2]. They are helped by a mesh of protein threads (fibrin) that holds the plug together [1]. If the plug is exposed to air, its surface dries and hardens. That dried blood, along with other fluid from the wound, is the scab.
So a scab is not a part of you that is growing. It is closer to a temporary lid, made mostly of dried blood, that was left sitting on top of the repair work.
2. Under the scab, skin cells are crawling across the wound
The real work happens below the lid. Cells from the outer layer of skin (the epidermis) move across the open wound to close it. One widely used reference describes these cells as moving over the new tissue forming in the wound "but staying underneath the scab" [2]. Roughly two or three days after an injury, other cells called fibroblasts also begin to enter the wound and start building new tissue [2].
When the new skin surface is finished, the scab has nothing left to cover. It loosens and falls off by itself. That is why a scab that comes off on its own usually leaves smooth pink skin underneath, while one pulled off early can take fresh cells with it and re-open the wound. How much damage early picking does has not been measured precisely in the sources I could read, so this part is the usual common-sense explanation rather than a tested number.
3. In 1962, a pig experiment suggested scabs might slow healing
For a long time the standard advice for a wound was to let it dry out in the air. In 1962 a researcher named George Winter published a short paper in the journal Nature on shallow wounds on the backs of young pigs. Pig skin is often used in skin research because it is similar to human skin in some ways. Winter compared wounds that were allowed to form scabs with wounds covered by a thin plastic film [3].
A later summary of Winter's study describes the result this way: wounds kept covered and moist were re-covered with new skin faster than wounds left to dry under a scab [4]. In other words, the scab was not the fastest route to healing. It seemed that skin cells could cross a damp wound more easily than a dry, hard one. One reference summary says a moist environment favours cell migration, while a dry wound produces a larger, tougher scab that cells must work their way around or through [2].
4. In 1963, a test on people gave a smaller, less clear result
A pig is not a person, so the idea was tried on people the next year. Hinman and Maibach made tiny wounds on the inner forearms of healthy volunteers. Some wounds were left open to the air and some were covered with plastic [4].
The original paper is only a two-page letter and I could not read its full text. A later summary of it says covered wounds were further along at day 5, but by day 7 all of the wounds, covered or not, had healed. It also says very few people took part and that results varied a lot from person to person. Because that account comes second-hand, treat the exact days as approximate. What seems safe to say is that early moist healing was a promising idea in 1962 and 1963, and that the human evidence from that one small test was thin.
5. Today, clean and covered is the usual advice for a cut or scrape
Later research and medical practice moved toward keeping wounds clean and moist instead of letting them dry out, and a common reference summary states that keeping a wound moist rather than dry makes healing more rapid with less pain and less scarring [2]. In the United Kingdom, the National Health Service advises stopping the bleeding, cleaning the wound and covering it with an adhesive bandage (a plaster, in the UK) or dressing, which helps stop it getting infected [5].
That is also why "don't pick it" and "cover it" fit together. Covering a clean wound helps protect the healing spot, and a moist wound is described as forming a smaller scab than a dry one, so there may be less hard crust to be tempted by. The 1960s experiments did not prove all of this on their own. They were the start of a debate, and later work built on it.
6. After the scab falls off, the skin keeps getting stronger
A scab falling away does not mean the repair is finished. New tissue keeps being remodelled for months. One reference summary says scar tissue is only about 20% as strong as its final strength after three weeks, and about 80% after twelve months, and that even then it does not quite match uninjured skin [2]. This number comes from a general summary page, so it is a rough guide rather than a precise rule for every wound.
That is a good reason to be gentle with a healing area for a while, even after the crust is gone.
7. When a wound needs an adult or a doctor, and what you can safely observe
Tell an adult if a cut is swollen, red and getting more painful, if pus is coming out, if there is dirt in it that will not wash out, or if bleeding will not stop. The NHS lists these as reasons to seek urgent advice [5].
If you already have a small scrape that an adult has cleaned and says is fine, you can watch it without touching it. Notice what day the crust appears, how its colour changes, and what the skin looks like when it falls off on its own. Never make a wound on purpose to study it. If you want to read more, the original 1962 and 1963 papers are listed below, and you can ask an adult to help you find reliable information about wound dressings.