First of all, the phrase 'Stop By' means 'To visit a person or place for a short time, usually, when you're going somewhere else, but passing by that place'. So, your sentence 'Stop by when you're around' means 'Come my house/room and visit me when you're passing by my house/ room.'

1. The sentence 'Stop by when you are around' means 'Come and visit me when you are near my home'. 2. The question ' Does it mean that he will always here when I don't know how to do?' doesn't mean anything. Did you get this from automatic translation ? Perhaps you should concentrate on mastering simple, clear English before you start worrying about more complex language. It doesn't matter if you don't know the expression 'drop by', but it does matter if nobody can understand what you are writing.