A natural refers to a player's "natural" expanding location from their starting base. Maps frequently make it easy for players to expand early to their natural location, even before they've built any units to defend with.
Naturals are usually easy to defend, albeit more exposed than a player's main. Frequently, attackers will have to go through the natural to get to the main. Some naturals, however, are in dead-ends behind the main base, where they are extremely well protected and extremely easy to expand to.