The document explains the past continuous tense and how it is used to describe actions that were happening at a specific time in the past. It provides examples of using the past continuous tense with phrases like "was/were verb+ing" as well as examples of using the past continuous tense alongside the past simple tense to describe two simultaneous past actions. The document also includes an exercise in the form of a mystery about determining who committed a murder by looking at what each suspect says they were doing at the time of the crime.