A hashrate is a measure of how many hashes per second an Bitcoin miner is performing. The higher the hashrate, the more chances the miner has of finding a block and receiving the block reward.
NOTE: WARNING: Bitcoin mining is a very complex process and can be difficult to understand for those who are not experienced in the field. Before engaging in any kind of bitcoin mining, it is important to understand what hashrate is and how it affects the process. Mining requires a lot of specialized hardware and software, and it can be dangerous if done improperly, so make sure that you are properly educated before attempting to mine.
The block reward is currently 12.5 BTC, so a miner with a high hashrate has a good chance of earning a lot of money.
The hashrate is also a measure of how difficult it is to find a block. If the hashrate is high, then it is more difficult to find a block and vice versa.