The phrase "to sign on" means to agree or commit to something. It can be used in various contexts, such as joining a group, endorsing an idea, or accepting employment. For example, one might say they are going to sign onto a petition supporting a cause, indicating their intention to add their name and support to the effort. In another context, someone may "sign on" for a job by agreeing to its terms and conditions, thus committing themselves to the position. The phrase can also be used in a more general sense, such as when referring to joining or participating in something, like signing onto an email list or registering for a class.