Passé composé vs imparfait

It would only be imperfect tense if the playing were some kind of background action. For example: Je jouais avec Elsa quand soudain elle a disparu. In this case the long-term action is cast in imparfait. If you're just telling me that yesterday you played with Elsa for hours, that's still a one-time, point-like event in the past.

Aside:

Next to the tenses, verbs have a second dimension which expresses how the action is/was happening (as opposed to when). This dimension is called the "aspect" of a verb and is a whole other grammatical topic. It is the aspect which determines whether to use passé composé, imparfait, or passé simple.

Did you know that English has an even broader range of expressing different aspects than French? Look at these examples:

I played with Elsa.
I was playing with Elsa.
I have played with Elsa.
I have been playing with Elsa.

All 4 are talking about me playing with Elsa some time in the past. They differ in the way time passes during the action.

In English, too, you would say: "Yesterday I played with Elsa for hours. Today I didn't play with her at all." Contrast that with: "Yesterday I was playing with Elsa when, suddenly, she disappeared." Or: "I have played with Elsa many times, but yesterday I played with her for hours." All different nuances of how the passage of time is perceived by the speaker.

Thank you for your response, Chris! Does the aspect topic for the verb come under a separate lesson here? 

I don't think that aspect has its own lesson. It is taught as a kind of sub-text with the tenses. You don't need to know tense or verb aspects explicitly as long as you develop a kind of feeling for them. And building on your own native language (assuming it is English), you already have a pretty good foundation.

There is this: https://progress.lawlessfrench.com/revision/glossary/aspect