JIT compilation has been one of the hot topics in PostgreSQL 11. It has been a major undertaking, and the first results look promising. However, let's start with the fundamentals: what is JIT compilation all about? When you run a query, PostgreSQL has to figure out a lot of stuff at runtime. When PostgreSQL itself is compiled, it doesn't know which kind of query you will run next, so it has to be prepared for all kinds of scenarios.
The core is generic, meaning that it can do all kinds of stuff. However, when you are in a query, you just want to execute the current query as fast as possible – not some other random stuff. The point is, at runtime, you know a lot more about what you have to do than at compile time (that is, when PostgreSQL is compiled). That is exactly the point: when JIT compilation is enabled, PostgreSQL will check your query, and if it happens to be time-consuming enough, highly optimized code for your query will be created...