One-sentence definition:
Mining hardware joined together and if a block is found then the reward is split between the miners depending how much effort was put in.
Why it matters: Pool mining matters because even smaller miners can join it, while solo miners put in a lot of effort to get a chance of 1 in 8 million to find a block.
Example: If several people are working together to mine bitcoin for example and they find a block, the reward is getting split between them based on the mining work done.
Links:
- Basics post: What is solo mining and how does it work?
- Related glossary term: Hashrate, Solo mining