What are invulns on team fortress 2

The Medic has a gun called an medigun that heals people. Over time, a bar will charge and at 100% you will have something that is called an ubercharge. Activating an ubercharge (right click) will render you and your patient invulnerable until the bar runs out.
Invuln is short for Invulnerable, when the Medic activates his UberCharge, you become Invulnerable. That's just a game term for it.