From Pokerstars FAQ.

Q How many play-money poker chips do I start with? What if I run out of chips?

A You will be given 1,000 play money chips to start. However, if at any time you are below 100 chips, simply click the dealer tray and select "Add More Chips...".

This "rebuy" feature is intended to replenish the stack of a player who has legitimately lost their entire stack while playing poker. The new 1,000 chips are for the use of that player only. Re-buys acquired for purposes other than this are subject to forfeiture.