Anything that might prevent something unpleasant or dangerous from happening is precautionary. If you're babysitting your neighbor's three year old triplets, you might want to hide every fragile thing in your house as a precautionary measure first.
If you do it beforehand to protect someone or something, it's precautionary. You may be sure you don't have a concussion after wiping out on your bike, but getting checked out at the emergency room would be a smart, precautionary move anyway. Putting on your helmet before you raced your bike down that hill was also precautionary. The Late Latin source of precautionary is praecautionem, "a safeguarding," from the Latin praecavere, "to guard against beforehand."