A warm-down is the set of easy, gentle exercises you do AFTER hard exercise — after a race, a game, a dance class, a gym session. Usually it means slow jogging or walking, then stretching, for maybe five or ten minutes, so that your heart rate comes down slowly instead of all at once and your muscles do not go stiff. It is a noun: 'do a warm-down', 'a proper warm-down'. You will also see the two words written without the hyphen, and as a verb phrase: 'warm down' ('The runners warmed down on the grass'). Two honest notes. First, in American English people say cool-down / cool down far more often than warm-down; warm-down is more of a British thing, made to pair neatly with warm-up. Second, sports scientists now argue about whether warming down really does much for soreness at all — but coaches and PE teachers still tell everyone to do it, so the word is alive and you will hear it. It belongs to sport and exercise talk; you would not use it about anything else.