Heroism allows you to quickly increase the health of one of your units of at least corporal rank (20 XP) to 200 for the cost of 4 TP. Useful for protecting your important troops from explosions and sniper assassinations, as well as for emergency healing. Heroism does not stack, and it cannot be done to vehicles.
Be advised that disabled troops cannot be "heroed", nor can dead units – obviously. Medics will ignore Heroism and still only heal that troop if it is below 100 health.
Medics can only be "heroed" if you use Combat Leadership or Crates to get them to Corporal.
Also keep in mind that both Sarge and Officers can use Heroism right away (Since they are both, technically deployed above the rank of Private First Class).
You can get one of these free every game with the Halloween Ribbon.