This is if you just let it sit in an account instead of investing it. The parent comment is saying if you invest it you will be able to take 4% each year and still keep the nest egg in tact.
You can search for "the 4 percent rule" or "trinity study" if you're curious about more. But basically, stock markets over long time average 7-8%. Subtracting inflation gives you a rule of thumb of 4% that historically would have worked.
Of course, the future might not match. But the studies also use a fixed withdrawal rate throughout. One could further mitigate the risk by having a small side-income some years, reducing spending in down-years etc. Also, how much money one needs post-retirement is often less, for instance if the mortgage is paid down, not housing kids any more etc.
