OpenGL consists of commands related to drawing 2D & 3D objects (e.g. draw a triangle, define material properties, define a texture).
Drawing takes place in some sort of window, controlled by an operating system.
OpenGL avoids including any sort of functions to create or manipulate windows, or to do other user interface tasks (reading keyboard/mouse, etc).
GLUT (GL Utility Toolkit) provides functions for windowing and interaction.
It defines a simple interface that hides the OS-specific details of these tasks.
GLUT functions can be used in the same way under Unix, MacOS, and Windows, making GLUT-based programs more portable.