Don't let this fool you, it's more often a trap than it's not.
My understanding is - communicate with the team - know the release dates - be working in critical moments...
Personally I would love to work 6 months a year or 3-4 days a week... To travel and to develop my side projects. Treating job as a way to pay bills in expensive city like London.
Unless - my personal incentives are aligned with the company - very often it's not the case.
It's because they want the employees to not take any days off. Peer-pressure is insanely strong in those environments. You are supposed to feel bad to letting your colleagues do the work, while you voluntarily (!) take time off. Especially at start-ups there is always something for you to work on.
As an employee it's easier to agree on like 25 vacation days/year beforehand because then it's in the contract and both parties can point to it when there are problems. If a colleague has a problem with someone taking more vacations days, it comes down to their own inability to negotiate more days for themselves; not to some "backstabber" leaving their colleague behind for some fun.
As I said, there are cases where unlimited vacation days work out but I would say those are the lucky 1%.
But either way -- just by calling it "unlimited vacation", they're f-ing with you from the very start.