Relative tense is a tense that refers to a time in relation to a contextually determined temporal reference point, regardless of the latter’s temporal relation to the moment of utterance.
- Walking in the woods, he saw a squirrel.
- Walking in the woods, he will find buried treasure.
Walking in both cases is understood as simultaneous with the time of the action of the main verb. This time may have different temporal relations with the moment of utterance.