Most poker tournaments are "freezeouts". In this format, you buy in at
the beginning and when you lose all your chips, you are out. But some
tournaments have a "rebuy" option. When you run out of chips, or even if
you just run low, you can choose to buy more chips. The rebuy amount is
typically the same as the buy in amount, though it can be less, and you
usually get chips at the same or better exchange rate as at the initial
buyin (for example, 1000 chips for $20). The rebuy period is normally
limited to the first few rounds of the tournament. At the end of the
rebuy period, there is often an "add-on" option. This is one last chance
to buy more chips before the tournament becomes a standard freezeout.
Usually all players are eligible to take the add-on, but sometimes the
add-on is restricted to just those players whose chip count is less than
a certain amount.
The strategy of deciding when and whether to make rebuys and add-ons is
an advanced topic in tournament equity. But good beginner advice is "when
in doubt, make the rebuy". You usually won't go far wrong. When you enter
a rebuy tournament, you should take this advice into account and plan for
the likelihood of investing more than the initial buyin.