The InnoDB storage engine maintains its own buffer pool for caching the data and indexes of its tables; this reduces the amount of I/O needed to access data from tables stored on disk.
The Buffer Pool Hit Rate alarm fires when the proportion of table data that can be found in the buffer pool drops below a specified threshold.
You may be able to improve the buffer pool hit rate by increasing the size of the buffer pool via the MySQL variable innodb_buffer_pool_size. The larger the size of this value, the less disk I/O is needed to access data in tables.
The MySQL Reference Manual suggests that, on a dedicated database server, you can set the pool size to be as high as 80% of the host's physical memory size. (A value that is TOO large may cause excessive paging in the operating system.)