All 3-dimensional graphics data are expressed in terms of a Cartesion coordinate system.
The coordinate system is a frame of reference - a location (origin) and 3 basic directions (coordinate axes). Everything is measured relative to this location and directions.
The OpenGL coordinate system is right-handed - +X to the right, +Y up, +Z out of the screen.
In a left-handed coordinate system, +Z would point into the screen.
The specific directions are arbitrary, and often differ in different software packages or application domains.