A Band Aid is a sticky gauze strip that's used to cover a scrape, cut, or small wound to keep it clean. Put a Band Aid on your mosquito bite and you won't be tempted to scratch!
Band Aids are made by the Johnson & Johnson company, although the word is commonly used to refer to any brand of adhesive bandage (unless you live in Britain, where they're called "plasters"). Skin your knee and you might need a couple of Band Aids! Figuratively, a Band Aid or bandaid is a quick, temporary fix for a larger problem: "Telling people to change their passwords is just a Band Aid on a major data security issue!"