You need to remove all of the cards from the pyramid.
Reveal cards from the draw pile, one, two or three at a time. If a card from the draw pile is not used, it should be covered up by the next card from the draw pile. It can be used later in the game, but only if it gets uncovered because any draw pile cards on top of it are able to be discarded.
When two exposed cards total 13, they can be discarded. (This is the reason that Pyramid Solitaire is sometimes referred to as Solitaire 13.)
NOTE: Discarding is always optional, and there may be times when it is a better tactical choice not to discard.
Cards in the pyramid are "exposed" if no cards are overlapping them. Only one card from the draw pile is "exposed" at any given time in the game (i.e. the most recent draw pile card to be turned up is the only card from the draw pile that's "exposed").