The text boxes in the upper-right corner explain when the function was introduced and some of the etymology behind that function. So if you see that a function was introduced into the OpenGL API in version 2.1, then you know it will be available in version 3.2, 4.0, or whenever. However, the function may have reduced functionality in versions lower than 4.3. For example, before version 3.0, glTexImage2D did not take integral image formats.
Documentation for all versions of OpenGL and GLSL is available at: