Solo mining is when a miner mines and validates blockchain blocks without joining a pool. Solo mining pits you against other miners to solve difficult mathematical puzzles and add a block to the blockchain. Upon success, you will receive the entire block reward.

Like playing a team sport alone, you don't share the win or prizes. Remember that mining in a pool, where participants share computing power to increase the odds of finding a block and split the rewards, can be simpler and less unpredictable than mining alone.