Черкаш is usually a rectangular spot of special substance on a matchbox, which you quickly rub matches against to light them up. Or something that is similar to that spot)
The other meaning of this word is a line which is left, for example on a wall or cloth, or whatever, after you've quickly rubbed a match or something else against it.
Also, there's a slang meaning of this word, this is the stain that is left on the back of underpants as a result of underpants' owner doesn't know what the word hygiene means
I hope that helps.