An out is a card that improves you enough. Four to a flush on the flop → about 9 outs left in a full deck (13 of the suit minus the 4 you see).
Rule of 2: on the turn, outs×2 ≈ percent to hit by the river. Rule of 4: on the flop, outs×4 ≈ percent to hit by the river. Estimates — not law.
Rule of 2/4 is a field estimate, not scripture. Count outs carefully and remove dirty outs that make you second-best.