Reinforce is a mechanic originally from Lorywn block, and lets you use cards to buff up your creatures.
Reinforce, always written as Reinforce N, is an activated ability that works when the card with Reinforce is in your hand. It essentially turns the card into a modal card. You can play the card normally and pretty much ignore Reinforce. Or, at instant speed, you can pay the reinforce cost and discard the card. If you do, you put a number of +1/+1 counters on a target creature equal to the Reinforce number on the card.
And…yeah. That’s really about it. There’s not even a lot of rules notes on this one. Reinforce can appear on any card type, and functions the same no matter which type it’s on.
It didn’t get a great reception, and the space of making cards have creature-buffing mode has been done again since with Bloodrush. But it is a useful mechanic, if not a terribly flashy one, and could show up again here and there in the future. Maybe not in a Standard set, but it’s already shown up in Modern Horizons 2, so supplemental sets are certainly on the table.
Want to Reinforce you collection? Get it? Reinforce?…Look, I’m sorry, please still get your cards at cardkingdom.com.