The phrase "to come on" can have several meanings depending on its context.
1) It could refer to a person or thing that is starting to become active, present or visible. For example, "the sun has started to come out after hours of rain."
2) It could also be used in the sense of something beginning to happen or occur. For instance, "The meeting will start soon so we need to get ready now."
3) In a more informal context, it can mean to begin flirting with someone or trying to attract their attention. An example would be: "He's been coming on to me all night long and I don't know how to react!"