shader is a thing that manipulate pixerls/colors (vertex shader) or vertices (vertex shader) to do transformations. to make shader you have to write it with special shader language (HLSL in example) and if you want to use it - use search
A shader is a program that modifies how the pixels/verticles are processed on the gpu. For example, say that you have a texture, on which there are 2 colors, say green and blue.
Now for some reason, you might want to filter the blue colors, so that only green and red are displayed(Altough there is no red in that texture.).
Then you could write a pixel shader, which takes the pixel you normally would display, sets the blue color part to 0, and returns the new color.
A vertex shader is pretty much the same, but works with the verticles instead, so you could reposition them or something like that.
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