When mining Bitcoin in a mining pool, we share the coin for percentage of the work we have contributed. But how is the work "proved". For example, if the mining software reported "it has done 1G hashes", how is it proved that the 1G hashes are actually done?
When a pool issues work to a miner, it is typically for a lower difficulty than the network difficulty. This allows the miner to solve a block header for the lower difficulty and submit it to the pool. These submitted headers are known as shares.
Even though most shares will not be valid for the network, having miners submit shares allows the pool to track how the individual miners are performing. Pools will pay their miners proportionally to the number of shares submitted, with some caveats (usually it isn't exactly proportional, there are various caps and other things calculations done, but in the end, the proportion of shares submitted is the major factor in the payout calculation).