It may mean not going out to eat all the time, not buying a fancy new car all the time, not having a fancy house in the most prestigious neighborhood, etc. I'm not saying it's bad to buy or want these things, but they are luxuries you pay for with hours in the office.
Early retirement is even easier if retire doesn't mean, "never work for money again", and instead means, "I don't have to take salary into consideration when deciding when/where to work". I'm an engineer living in the SF Bay area who doesn't make tech money and I'll still be able to retire after 15-20 years, easily, while still having waaaay more than my parents did when I was growing up.
I feel like this isn't true at all for retiring prior to age 50.
You have to adjust for cost of living and the area's median income. The median income in my state is $60k-65k and my salary is about $85k. I'm a midlevel dev in the industry for 9 years (no way could I retire at 15-20 years, even for a lower paying job). I also have a family to support. I do live within my means, but that doesn't mean much with the moderately high cost of living in this area.
There are some bonuses and profit sharing that might bump it $5k-15k. The housing market here has been hot for years now. Even with a long commute a builder grade house in the 1500-1800 sf range on less than an acre is over $250k with about $4-5k in taxes. It's about double that price if the commute is half as long. As an idea of cost of living, labor charge for stuff like a mechanic or plumber is about $100/hr.
In Seattle with a short commute you'd pay $900k-1M on a 1500-1800 sf house on 1600-3000 sf of land depending on if it's a townhouse or 1930s construction. $100k is a starting salary for new grads here. And plumbers/mechanics still charge about $100/hr, I think. (I have never had need of a professional plumber -- I do it myself -- and haven't had to do any auto work beyond oil in a few years.)