This sentence stands for the third person singular in the imperfect tense. 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. So, the word vivía comes from the verb vivir (to live). Since, vivir is an ir regular verb, If you want to conjugate a regular verb in the imperfect tense in Spanish for the third person singular, you need to remove the infinitive ending ir and add the ía to the end of the word. So:
Rosa vivía en Londres translates into Rosa lived in Londres
