RE = Retire early = retire from the necessary but emotionally unfulfilling jobs. For most people RE means pursuing interests that are not financially viable if you aren't already FI. It can mean a youtube channel, studying whatever you want or working on a side project without strict deadlines or the stress that comes with an all-or-nothing endeavor.
The key fallacy in FIRE, is that it ignores creeping costs and the stubbornness of your dependants. Avoiding lifestyle creep in fundamental to the movement. This also means being unable to fund fancy private schools for your kids and your spouse being fine with the sudden loss of a huge income source. FIRE is an empty pursuit without a definite end goal. If you don't know what you want to do during RE, you might just be condemning yourself to a loss of purpose and possibly significantly higher risk of death/mental deterioration.
Ofc, you can always pursue a more relaxed form of FIRE. Make enough to move to part time / contracts, or move to another country with lower wages or fully dedicate yourself to a moonshot.
The basic assumption generally being, anyone who can save two-thirds of each paycheque and invest in a portfolio yielding 4% per year after inflation can retire in 10 years. Saving two-thirds of each paycheque is of course difficult unless one is in a high pay grade to begin with, but there are enough people with the necessary discipline to keep the dream alive for many of us.
What's FIRE in this context?