It means having investments producing enough money to cover your living costs, so that you don’t need to work.

Most people are familiar with this idea in old age with a pension, but the FIRE movement (“Financial Independence – Retire Early”) tries to reach FI sooner, through a combination of efficient investing and saving often involving practicing frugality to reduce your living costs. The “Retire Early” part is optional, but in any case people value the freedom of being able to give up work if they wanted to.