generally speaking, -ish ending means "something like", or "a version of", but it really depends on the context. It's like watering done the effect of the word. Tallish loosely means "not very tall, but taller than average"; brownish means "slightly brown"; thirtyish would suggest "about thirty".
-ish ending cannot be used with any word. Well, actually it can be, but there's a degree ranging from perfectly grammatical to totally ungrammatical