A dab is a quick, soft application of something, like paint or makeup. You might touch up a clown's makeup with several dabs on his cheeks and forehead.
A dab is the motion with which you pat or apply something to a surface, and it's also the tiny amount of material you're applying. An artist might work in dabs of paint, and a baker might lick a dab of cake batter off her finger. Dab is a verb too: "Dab this aloe on your sunburn — it'll feel much better." A dab is a "light, striking movement," from the earlier dabben, "to strike."