OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Debug Output

The primary feature of a debug context is the ability to perform additional error checking and analysis. There are two ways that a debug context can get this information back to you. The first, which is really intended to support remote rendering is via a log that is part of the context and must be queried. The second, and far more efficient mechanism is through the use of a callback function. The reason that a log exists for remote rendering is that a remote server cannot call a callback function in the client application. However, whenever direct rendering is in use, it is strongly recommended to use the callback function.

A callback function is essentially a function pointer that you pass a third-party component (such as OpenGL). ...