a shield: the hand-held piece of armor a soldier holds up to block blows, and by extension anyone or anything that protects
At its core this is the warrior's shield, the small, often round piece of defensive armor a fighter carried on one arm to ward off arrows, spears, and sword strokes. It is the most common shield word in the Hebrew Bible, and most often it means exactly that: a real weapon of war, counted in armories, captured by enemies, and brandished in battle.
From that concrete picture the word reaches naturally outward. Because a shield is the thing that stands between a person and harm, it became one of the Bible's favorite images for God: the LORD is the shield of his people, the cover they hide behind when danger comes. This is the same word used in the same way, not a separate term; the poets simply point at the soldier's shield and say, that is what God is to us.
The same logic can stretch one step further to the human protector. A king or leader was thought of as the shield of his people, the one whose strength kept them safe, so in a few poetic places the word seems to stand for a ruler or sovereign rather than a literal piece of armor. This last use is debated, and the line between God as shield and the human protector as shield is not always sharp.