The theory box describes the Past Continuous tense, which is used to describe:
a) An action in progress at a specific time in the past.
b) An action in progress when another action interrupted it.
Examples from the text:
* "He was watching TV at 6 o'clock yesterday afternoon." (action in progress at a specific time)
* "He was cooking when there was a power cut." (action in progress interrupted by another action)