Turn on is a phrasal verb with a number of applications in formal and informal contexts. To ‘turn something on' usually refers to the act of switching on something mechanical or electrical to start it or to get it to work. You can ‘turn on' the oven, ‘turn on’ the tv or 'turn on’ the lights. If you 'turn on someone', you physically or verbally attack them, or treat them badly or aggressively. This is usually sudden or surprising behaviour. If you 'turn somebody on', it can mean that you excite them sexually. You can also be 'turned on by’ someone in the same manner. This is a casual or informal use of this phrasal verb that you might hear in conversation with friends.
Additional examples with 'turn on' are:
You are explaining the steps of a recipe to your friend: "The recipe says to turn on the oven first to pre-heat it."
Last night your friend behaved in a way that was very out of character: "We were all having a great night when Jamie just started getting angry, and he turned on us all. He said some very nasty things."
You are explaining why you like Latin music: “I don’t know why, but it just always turns me on!”