The ability to survive outside / without a roof over one's head.

I'm not talking surviving solely off the land type of outdoor survival, although that is an excellent set of skills to obtain.

Learn how to set up shelter, or find shelter if a tent / tarp isn't available
Learn how to start a fire in multiple ways
Learn basic hunting skills
Learn how to sanitize / store drinking water
Learn basic first aid
Always remember the order of operations in a survival situation: Shelter, water, fire, food