Leían is the conjugation of the er verb leer that matches the third person plural in the imperfect tense. In general, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. If you want to conjugate a regular verb in the imperfect tense in Spanish, you need to remove the infinitive ending (in this case er) and add the imperfect ending that matches the subject. For the third person plural this ending is ían. Finally:
Los abuelos leían el periódico.
