A shield is a cover that protects whatever is behind or beneath it. If you carry a shield you might be a knight, or a traffic cop who likes to flash the badge.
Ancestors of the word shield include the Old English scild, similar to sciell, or "shell," which suggests the meaning of the word. If you're a turtle, you might use your shell as a shield. But a shield doesn't have to be something physical. Philosopher Soren Kierkegaard once said, "Love is my sword, goodness my armor, and humor my shield." The word can also be used as a verb. You might shield your tender ears from hearing naughty words. Don't worry, shield is perfectly safe.