A precaution is a careful action you make in advance. You might want to take the precaution of bringing lots of water and sunblock if you're going on a desert hike.
Precaution means exactly what it sounds like. The prefix pre- means before, and caution means carefulness in the face of danger. People use glasses when they're on a computer as a precaution because there are links to long hours on a computer and vision damage. A fire drill is a precaution so that you know what to do in case of a real fire. Someone who is always careful exhibits precaution.
