Convoke lets your creatures help pay for a spell. While you're casting a spell that has convoke, you may tap any number of your creatures, and each one you tap pays for {1} or one mana of that creature's color. You can mix that with mana from lands or other sources, so a creature only covers part of the cost if that's all you need. The creatures aren't sacrificed — they're just tapped, and they untap as normal on your next turn.
In play, convoke turns a wide board into a discount. A deck that commits many small creatures can cast an expensive spell several turns earlier than its lands alone would allow, and those same creatures can still attack or block on other turns. The trade-off is timing: tapping your team leaves you exposed, so convoke rewards planning around when you commit your board.
