Someone who is agitated is visibly upset. If you have ever walked into a room and instantly been asked, "What's wrong?" you probably looked agitated.
The verb agitate means "to shake up." So someone who is agitated has been shaken up by something — disturbing news, a careless driver who nearly caused a huge accident, or just a very long, very bad day. Agitated can also describe being activated, or fired up about something. For example, a political cause might be losing steam until an event like a rally gets everyone agitated all over again.