You would call out to JNDI in a logging statement. That would cause the remote class to be loaded and executed during the log statement evaluation.

A nefarious attacker could inject such a JNDI reference in a field (like username or whatever) and if you wrote your log statements in a manner that didn’t expect such injection to happen, it could become part of the log format instead of a log field value, and this would be executed.

Think of it like SQL injection but with log statements and way worse because it calls a class that can be hosted on a server of choice. And the code that can execute is arbitrary and not limited to the database.