The past continuous can be used to describe what happened in the past.  For example I was speaking.  In English we often use this tense to say what happened in the past e.g. I was working on Friday.  

This tense is formed by using the imperfect part of the verb Estar plus the present participle.  The present participle is formed from the stem of the verb plus ando for AR verbs and iendo for ER and IR verbs.