Okay, I think it'll be best if I explain that with an example.

Imagine you have a very good friend wo knows-it-all about a particular subject, say - computers and you yourself know little to nothing about them. So one good day as you have been using your computer it suddenly conked out on you and then it refused to start up again. You got very upset and started shouting swearings but as you were doing that someone knocked on yoour door, you opened it and saw that it was your very know-it-all about computers friend. Now is when you say something like that:

Ты очень кстати! - Ты так кстати! - Очень кстати!

You do that because you know that your friend now can help you fix your computer, more importantly he had come just at the best time it could ever be
I think a proper English equalent would be - ""What a timely arriving!". At least I sure have heard that one before

But make no mistake, кстати is not only about someone arriving at the best time for you It's just about anything:

Just need to nail a nail, but you have no hammer? Fortunately, someone else just happened to have one around? - Очень кстати!
Wanted a promotion for ages and now your superior just got fired? - Очень кстати!

and anything like that

Hope it helps