The preterite and the imperfect are two tenses in Spanish that are used to talk about actions that occurred in the past*. However, they are used in different situations.
The preterite is used to describe an action that occurred at a specific time in the past and has already ended. For example: "I went to the supermarket yesterday" (I went = past tense).
The imperfect, on the other hand, is used to describe an action that occurred on a regular basis or had not been completed in the past. For example: "When I was a kid, I always played football on Saturday afternoons" (I played = imperfect).