Depth can describe many different concepts both abstract and concrete, and such concepts can be either qualitative or quantitative in nature. Most common is a reference to how "deep" in distance from top to bottom or inside to outside (thickness) something is.

In photography/ optics the depth of field describes a range (in length) at a distance from the lens where objects are in focus.

In mathematics depth can be any of a wide variety of concepts such as a regular sequence, depth of a graph-tree, number of steps on a topolologic path, depth in 3D geometry (height, width, depth), etc.

simple:Depth