A strong password is a random password. By "random" I mean: "whatever process produced that password could have produced a lot of other distinct passwords". "Entropy" is the name of the mathematical concept by which this randomness can be expressed. Take care that "password entropy" is a property of the process which generated the password, and cannot be measured on the password itself (regardless of what so-called "password meters" may claim).
This question is the classic reference here; see in particular this answer for what "entropy" means, and that answer for an important meta viewpoint, namely that a good password must have high entropy but also be easy to remember by human beings.