This tutorial series will walk you through writing your first shader. It is intended for people who have very little prior experience with shaders and want to get started with the basics. This tutorial will not cover advanced topics and it is not comprehensive. For a comprehensive and detailed overview of shaders in Godot see the Shading Reference Page.
"What are shaders" gives you a high-level overview of what shaders are and how they fit into the rendering pipeline.
The "your first shader" tutorials walk you through the process of writing a shader step-by-step.
For a more general introduction into shaders and the OpenGL Shading Language, use The Book of Shaders.