Stale rate is number of shares submitted after the previous block has already been found and pool has moved to the next block. If everything works correctly, it should be a very low number. Several factors can increase your stale rate:
On slower networks (or when there is a network issue on the path from your miner to the pool), it takes more time for a miner to receive notification about a new block.
However, the miner still keeps submitting results to an old block for some time and these are being rejected by the pool as stale shares.
Slower Mining Device
Some miners are like men. They only handle one task at a time. Before taking on a new job, they need to finish the previous one. While others are generating shares for a new block already, these miners are producing shares for the old block that cannot be accepted by the pool anymore. Such results are also called stale shares.