Hi Paula, It's a very casual way of saying "to speak fluently" and the meaning is derived from an idiom "чесать языком" close in meaning to "to have a chinwag". In some other cases, чесать could also mean "to lies / stories" as in "кому ты чешешь!" (don't lie to me, do you think i'm an idiot?) Hope this helps, Ivan.

the guy meant that your russian is pretty good. it is some kind of slang that mean 'speak'. it meant well.

Also it may be used in imperative. «Ну, почесали!» means "Well, let's go!"

here it's a slang word for "speak"

This word means "speak untruth", it`s street slang