To inject is to give an injection, or a shot. When your doctor tells you that you need a flu shot, she wants to inject you with a vaccine that will help keep you from getting sick.
You are most likely to hear the verb inject at the doctor's office. Veterinarians inject dogs each year with rabies vaccines, and sometimes if you're sick enough, a doctor or nurse might need to inject you with antibiotics. You can use inject in a figurative way, too: "I'd like to inject a new topic into this conversation, it's getting so boring." The Latin root is inicere, "to throw in" or "to throw on."