Cover

Sometimes during a fight, you might seek cover, such as by diving behind a small barricade or ducking behind a tree. When you take cover behind something that makes attacking you more difficult (but not impossible), attack rolls against you are made with disadvantage.

You usually can’t be targeted if you’re entirely behind a substantial structure like a wall, even if you’re technically in range of the attack. However, you might still be affected by nearby explosions or similar effects that target an area.