Refers to a type of cards whose purpose is playing other (usually more valuable) cards from your deck. Tutors provide consistency by thinning your deck as well as sometimes giving you direct access to your win conditions. Bricking can often happen with this type of card.
Example: “Fauve” is a tutor card because it can play any card with the “Nature” tag from your deck. So if you play, for example, “Nature’s Rebuke” through your Fauve, you will have tutored your Nature’s Rebuke with Fauve.