For a miner, the probability of finding the hash is equal to their portion of the total mining power on the network. This means that miners with a small percentage of the mining power stand a very small chance of discovering the next block on their own.
Mining pools are created to solve this problem. Mining pools are gathering / pooling of resources, by miners who share their processing power over a network. These pools are splitting the reward equally among everyone in the pool, according to the amount of work they contribute to the pool and as a result to the probability of finding a block. With this model, every miner, smaller or bigger, earns a specific number of shares and gets a percentage of the block reward when the pool is finding the next block.