Bitts and Bollards are heavily built devices for belaying lines. Bitts may be classified as single, double, H-Bitt, Cruciform Bitt and others.
Bollards may have a single post, double posts (either vertical or angled outward from center), Cruciform, staghorn, and many other styles. Sometimes Bitts are called Bollards and vice versa.
Friction is necessary to keep lines attached to Bitts, Bollards, Cleats, Lines, or other things. Friction increases as the "normal force" increases. The normal force is the perpendicular force between two objects. When the load on a line increases, say by the boat drifting away from the dock with the line tension increasing as a result, the normal force between the line and the Bollard increases and therefore the friction increases. The connection will hold if the total friction between the wraps of the line and the Bollard is greater than the force on the line to the boat, otherwise it will slip.