To reduce the area of one or more sails while they are being used by winding them over a roller or tying them up at particular points. Often specified as "the mainsail," "the jib," etc. There's too much wind to be at full sail—it's dragging the whole boat to one side. I'll keep a steady course while you go and reef the mainsail. We should be fine if we just reef the jib, I think. The captain ordered us to reef the sails in before the storm hit.