Total Cover

A target has Total Cover when it is completely concealed by an obstacle, such as a wall, a thick tree, or a similar barrier large enough to cover it entirely.

A target with Total Cover can't be targeted directly by an attack or a spell, even if part of its space is within range or reach.
Total Cover also blocks line of effect: an area of effect (such as a Fireball) only includes a location if a straight line from the point of origin to that location is unblocked. An obstruction that provides Total Cover blocks that line, so a creature fully behind it isn't caught in the area.
A creature can provide at most Half Cover to a target; it can never grant Three-Quarters or Total Cover.

If a target is behind more than one degree of cover, it benefits only from the most protective degree. For reference, lesser obstruction grants Half Cover (+2 bonus to AC and Dexterity saving throws) or Three-Quarters Cover (+5 bonus to AC and Dexterity saving throws).