Rendering is the most essential operation within a video game. However, it is not a cheap process, as it requires sending data from memory to the GPU as well as a sequence of steps within your GPU to transform the input data into the final 2D image.
This input data consists of vertices that are no more than points with extra attached information, such as texture coordinates. They will serve as a base for constructing basic shapes.
The following is a summarized and simplified screenshot about this process:
Here is a brief but concise explanation: