OLAP means Online analytical processing. This database structure approach is not based on the use of a transactional database, it is not optimized for many small read and write operations, but it is specifically designed for data visualization and reporting systems (eg Metabase) that are able to serve optimally multiple, large, combined queries.
The term comes from the "Online Transaction Processing" database schema approach, which focuses on database transactions to being able to perform many writing, reading, updating operations quickly, simultaneously, and stably.