A turn-on is something that excites you, and in nearly every real use, sexually. It is a countable noun: "Confidence is a real turn-on." "That's a big turn-on for him." The opposite is turnoff (or turn-off), something that kills the attraction: "Bad manners are a total turnoff." Turnoff is actually the more useful of the pair for a learner, because it has stretched into ordinary non-sexual use — people say a boring teacher or a long form is a turnoff, meaning it makes you lose interest. Turn-on has stretched far less; it stays mostly sexual, so if you say something at work is "a real turn-on" people will hear the sexual meaning and it will be awkward. This is the honest, useful point about the word: recognise it, be slow to produce it. Note the shape carefully. The noun is written with a hyphen or as one word, turn-on / turnon, and the stress goes on TURN. The verb is two separate words with no hyphen: "You turn me on." Do not confuse either of them with the everyday verb turn on meaning to start a machine — "turn on the light", "turn on the air conditioner". Same two words, completely innocent, and by far the most common meaning; only the noun form carries the sexual sense. Register: casual spoken English, magazines, dating talk. Not for formal writing.