Basically, debridement is the removal of dead, dying, and infected tissue from wounds to facilitate healing. It looks gruesome, but the process leaves behind only viable pink tissue that is capable of healing itself. In Kelly's case, that means scraping away all the discolored, weeping flesh along with the thick black scabs of dead tissue. The thick black crusts you see can pull the flesh under them very tight, restricting blood flow and causing more tissue death.
Most people are familiar with it within the context of burns. The burned flesh needs to be pulled away from the living flesh so body can begin healing. This process can be hideously painful for the patient. Maggot therapy is also fairly well known, where sterile maggots are used to clean dead flesh away from the inside of wounds. Surgeons can also go in and mechanically debride woulds with scalpels and other tools. Your body will do this on its own, to a certain extent, when dead skin falls away from cuts and other minor injuries.
Kelly is doing her own kind of debridement with her constant picking. She's reaching in there with her dirty fingers to directly remove flesh, spreading more infection and preventing healing. The healing flesh is so fragile that she's killing it as soon as it can grow in. In a sense, she is eroding her own legs away, one pinch at a time.