What does the 'strength' mean (in numbers)? The coffee package I've bought has 'strength rating'. It's 3 out of 5. 

I wonder what that 'strength' means? How it is measured? Does it relate to the caffeine amount?

The coffee package is from Germany, and from what I've seen, all coffee sorts have this rating, so it's a kind of standard.
Coming from Germany myself I have seen this rating only on a single brand of coffee. There is no standard for this (even though us Germans seem to like standardizations), but basically strength of the coffee is about the inherent bitterness and caffeine content you get out of it.

The stonger a coffee the more caffeine and also the more bitterness you will usually get.