What is MySQL's `innodb_buffer_pool_size` and why is it important?

`innodb_buffer_pool_size` is the amount of memory InnoDB uses to cache data and indexes. It's crucial for performance. A properly sized buffer pool can dramatically reduce disk I/O. Insufficient memory allocation will lead to frequent disk reads, slowing down operations significantly. Generally, allocate 70-80% of available RAM to this buffer pool, but adjust based on workload.