A shader is a program that executes on your GPU. It gets some data as an input and it outputs some data. That's all there is to them.
When opengl renders stuff it goes through various passes, during which it may interact with the currently active shader program. An interaction in this case would be opengl passing some data to the program and collecting the results.
Write one, bind it before rendering and you are done.
Of course shaders are fairly complex and difficult to discribe like that. There are many resources online that introduce you to shaders. Just keep in mind that minecraft isn't exactly shader friendly, as in it's rendering engine is a huge mess of pre 3.0 and post 3.0 GL code.
https://learnopengl.com/Getting-started/Shaders is a good one I am personally aware of, but apart from that just google "opengl shaders tutorial" and I am sure you can find a lot of information out there.