A subculture of frugality & intentional living, short for “Financial Independence, Retire Early”
FIRE is an acronym (actually an initialism, but whatever) for “Financial Independence, Retire Early”.
These people aggressively save and aggressively cut their cost of living so that they can be free from the need to for employment as early as possible. They engage in things like house hacking and can tell you all about Tax-Advantaged Accounts and investments.