Camping recovers some hit points, spell points, and endurance points when used. Rangers are particularly skilled at camping, but everyone can find some use for this skill. High camping is one of the better ways to regain endurance points in particular. Camping is about half as effective when used indoors, unless paired with a high amount of mastery of camping. This is the skill the ‘sleep’ command activates, even if you are untrained in it.

A sleeping bag will improve your results.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: Con.
It uses no endurance points.
Living in the wilderness is a tough way to survive. 
Often, there is little time for one to recover from the rigors
of finding food and fuel. Being able to rest one's self is
a discipline in itself. By learning how to relax outdoors and
take a short catnap, one can regain lost vigor and strength.
There are many ways to make these rests even better. Those 
who are skilled at this can also use this when not in the 
wilderness, of course there is nothing like being in the great
outdoors.