When you walk, you watch out for nails and holes so you don't hurt your foot. Do the same thing with your mind — protect the part of you that makes decisions. If you guard your thinking in everything you do, you'll act more safely.
As in walking you take care not to tread upon a nail, or turn your foot, so likewise take care not to hurt the ruling faculty of your mind. And if we were to guard against this in every action, we should enter upon action more safely.