Pool mining works like this:
Each miner connected to the pool will repeatedly ask for a chunk of work from the server. The work provided is a much less difficult piece of work, which results in the miner being able to submit "shares" at a relatively quick pace (in comparison to the time it would take to solve a block solo).
Each share could potentially be the answer for the block the pool is working on, and if it is, the pool solves the block and each user gets paid based on the method the pool is using to value shares. Less any required fees or donation rates.
Using bitcoinpool.com as an example, each share is paid as a proportional amount in regard to the whole. For example, if 100 shares are submitted before the block is solved, and you submitted 1 of those, you would get paid ((1 / 100) * 100) / 2. Or, 0.50 BTC.
Essentially, multiple users are working together to solve a block at a faster rate, but for less payout per block than they would receive while mining solo.
If it takes your card 4 weeks to solve a block solo, you would receive 50BTC for solving it, but only solve 1 block per month.
If you participate in a pool that solves a block once per day, and you receive a 2BTC payout per block solved, in the same 4 week period of time, you would get 56BTC, but it would come in much smaller daily payouts.
Essentially, you're trading singular high-payouts, for multiple small payouts in much quicker succession.
I hope that helps!