That term refers to a low CMC card, usually 2 mana
Mark my words though: If we see the return of Learn in the next Strixhaven set, everyone is going to have at least one Aang's Journey in their sideboards, and probably two
It comes into play untapped and taps for any color, but it comes with the huge drawback of passing to your opponent on your end step if you use it
(My Equinaut Deck is a deck based around Equilibrium and Fleetfoot Panther that plays and replays creatures with self-bounce triggers to abuse various triggers like Aura Shards, the bounce trigger, or tap them before they had back with Opposition, and so forth)
There are a lot of cards that can cycle for just one mana (and in some cases, for free), letting you deal a lot of burst damage together
Five out of five stars, would recommend