Poder in the imperfect merely expresses the ability to do something. It doesn't express whether it actually happened or not. So if I say, Ayer podía ir al cine, all you know is that I had the time off and the money to go to the movie. Maybe I went, maybe I didn't — neither is implied. In the negative, Ayer no podía ir al cine, though, we know I didn't.
On the other hand, in the preterite, poder expresses a bit more. If you say Ayer pude ir al cine, you were able to and you did. In the negative, if you say Ayer no pude ir al cine, you are indicating that you attempted to go, but failed to, and therefore you did not go.