Synonyms: First Aid, Cure
Heal is a Prowess, this special kind of Feat can't be acquired until the third level. A Prowess is a special action that can only be done once per scenario. Unlike normal Feats, it can be acquired several times.
To the extend of the humanely possible, you miraculously cure one disease, poisoning or wound, no matter how severe on you or someone else.
For example, it is humanely impossible to heal someone who has been decapitated, but it is not impossible to treat someone who's throat have been slit. It is very difficult, very rare, but humanely plausible.
You have to take the time to treat the wound and be physically able to move and act in order for this Prowess to have effect. It is not reasonable for example to treat yourself if your own throat was slit.