The phrase "to be on" has several meanings depending on its context. Here are some common usages:
1) To be in a particular state or condition, such as being on vacation, being on duty, or being on call. For example, "I am on vacation next week so I won't be able to attend the meeting."
2) To have an opinion about something, such as being for or against it. For instance, "He is always on about the importance of education."
3) To support a particular cause or movement, such as being on board with a new policy at work. An example would be, "I am not fully on board with this idea yet."