If you want more info look here: https://ubuntu.com/security/esm

Imagine if you were trying to keep compliance and suddenly a security update was an ESM package found in the Universe repo? To get that update installed you’d have to pay Ubuntu Pro for each host you have.

They have a personal license like how Red Hat allows some licenses for personal use so if you have a few home machines it’s fine. But if you have a job that has a fleet? Not a fun time to suddenly need to pay for updates even during LTS window.