The OpenGL Shading Language (GLSL) is the standard programming language for shaders in OpenGL
HLSL(High Level Shader Language) is a proprietary shading language developed by Microsoft for use with the Microsoft Direct3D API
Either way finding hlsl shaders is harder than finding glsl shaders opengl is kind of a universal thing.. which can then be ported to hlsl
How to use or make "Effects" in monogame?
Opengl is better for playing around and experimenting because you can easily draw triangle without messing with hardware buffers available on larger selection of platforms plus glsl is better than hlsl because it doesn t compile into assembly
Creating effects in 3D game from scratch, using c++ and DirectX