To truly understand OpenGL you need to know three main concepts:
- Linear Algebra
- OpenGL API
- OpenGL Shading Language (GLSL)
The main things that you need to know in Linear Algebra are: Vector operations, Matrices operations, Transformations. A Transformation allows an object to be rotated or translated on the screen.
After you have good understanding of Linear Algebra, learn how OpenGL transfers data to the GPU. The whole purpose of OpenGL is to transfer data from the CPU to the GPU. Learn what is an OpenGL Object , Vertex Buffer Objects , and Vertex Array Objects.
Once data is in the GPU, then it can be drawn on the screen. This is where the OpenGL Shading language comes into play. Learn about Vertex Shaders and Fragment Shaders . These shaders are responsible for positioning and applying color/texture to a mesh. A mesh could be a game character for example.