Walls, trees, creatures, and other obstacles -- collectively called cover -- can make a target more difficult to harm. There are two degrees of cover (described below). If a target is behind multiple sources of cover, only the most protective cover applies; they aren't added together.

Partial Cover: Fighting tests have disadvantage against a target that has partial cover. A target has partial cover if an obstacle blocks at least half of its body. The obstacle might be an object, a feature of the environment, or the space of another creature (whether that creature is an enemy or a friend).

Total Cover: A target with total cover can't be targeted directly by an attack or other effect. A target has total cover if an object or a feature of the environment blocks its whole body. At the GM's option, unusual circumstances can cause other situations to count as total cover.

Adjudicating Cover on the Grid: To determine if a target has cover from you (or from an effect), choose a corner of a square you occupy (or the effect's point of origin) and trace imaginary lines from that point to every corner of any one square the target occupies. If two or more of those lines are blocked by an obstacle, the target has -- at minimum -- partial cover. A line isn't blocked if it merely follows the edge or nicks the corner of a square that contains an obstacle.