Also known as sequence-of-tense rule.
In English grammar a backshift is the changing of a present tense to a past tense following a past form of a reporting verb.

I told Stacy that Kim had blue eyes.

My utterance to Stacy would have contained the present tense form “has”, but my report of it contains preterite had. Nonetheless, my report is entirely accurate. This kind of change in tense is referred to as backshift.