To use all of the community cards in a flop game such as Holdem or Omaha to make the best five card hand. If no one has a better hand utilizing any of their hole cards then the pot will be split evenly between all the players in the hand.
To make your best hand using only community cards.
In hold'em, using all five community cards when your hole cards don't make a better hand. If you play the board, the best you can do is split the pot with all remaining players.