Up or out is a term used to describe jobs/positions where you either rise up the ranks or get the boot. This is an article written together with terminal jobs, so go check that out.
You're almost always in up or out mode as a junior in a company. You don't have strong skills, and the first 5 years of your career are going to be the years you advance the fastest. Because young people are less skilled and needed, you're often the first on the block. It's also true many high end, prestigious firms also have up or out for most your time there. Law firms, banks, consulting firms, and most jobs that are considered "accelerators" to a career have you working hard, and usually high work hours means high chance of being up or out. I'm not sure why this is, but generally long working hours have high correlation to up or out.
A major advantage is simply that you advance very quickly. Going from junior to senior employee in 6 years is a pretty fast track. You also usually make better money and have a lot more opportunities to learn. Learning is super underrated, because knowledge in businesses is the same as money, and if you make more money for a business, you have a lot more negotiating power. Now, the disadvantages are also very apparent. Long working hours and constant stress are real problems. Because you always need to perform to climb(or get booted), you're going to feel the stress of performance. You also have the major problem of a lack of WLB. This is fine for some, awful for others.
This is actually much easier than getting into up or out. Very simply, you need to work for a few years, then very simply settle in. If you read the terminal article, you'll notice what level is terminal. If you just get to this point, the pressure generally eases off. Also, join a new company. There are a lot of less advertized, less attractive companies out there that will have much better WLB. Working as a SWE at a top FAANG/Quant firm? Well, just go to a background company like Oracle or Intel, or a company with strong family values like Disney.