A share is awarded to miners who submit a valid proof-of-work to the mining pool.
There are two types of shares - accepted and rejected. Accepted shares represent work done by the miner, and the miner will be rewarded accordingly by the pool. Rejected shares represent work that does not contribute toward block discovery, and therefore the pool does not provide a reward.
The most common reason for rejected shares is latency issues at the farm, causing the mining machines to work on blocks that have already been mined on the blockchain.
The rejection rate is the proportion of rejected shares of the total shares submitted, and therefore the lower the rejection rate, the better.