I guess you could say it's an alternative to postgres. It's a different database, that's column oriented which makes different tradeoffs. I'd say DuckDB is a better comparison, if you're familiar with it.
Expanding for the original question:
Roughly speaking, Postgres is to SQLite what Clickhouse is to DuckDB.
OLTP -> Online Transaction Processing. Postgres and traditional RDBMS. Mainly focused on transactions and addressing specific rows. Queries like "show me all orders for customer X".
OLAP -> Online Analytical Processing. Clickhouse and other columnar oriented. For analytical and calculation queries, like "show me the total value of all orders in March 2024". OLTP database typically store data by column rather than row, and usually have optimizations for storage space and query speed based on that. As a tradeoff they're typically slower for OLTP type queries. Often you'd bring in an OLAP db like Clickhouse when you have a huge volume of data and your OLTP database is struggling to keep up.
What's the significance of "online" in these acronyms?
Online means you expect the responses to come quickly (seconds) after launching the request. The opposite is "offline" where you expect the results to come a long time after making the request (hours / days).
ClickHouse is designed so you can build dashboard with it. Other offline system are designed so you can build reports that you send in PDF over email with them.
It is a rather old acronym. The other option was batch processing, you will get your results in the mail type thing.
Here "Online" means results while connected to the system, not real time since there is no time requirement for results.