In computer graphics, a shader is used to color something. Traditionally, shaders were so called since they defined the shade that an object got based on its original color and light source position.
Nowadays, shaders aren't really thought of as providing shading to an object as much as a textured, final color.
Note
Shaders are about determining the final color of an object given the light source, geometric positions, and initial colors (including textures, and more expensively, material properties).
There are two flavors of shader: vertex shaders and pixel shaders.
Vertex shaders: Color at the vertex (point in the mesh), and smoothly shade from one 3-space point to another 3-space point.
Pixel shaders: Color at the pixel (point on the screen). The 3-space physical location of a pixel (aka fragment) is calculated using some simple math.
In UE4, we just call a shader a Material. Materials abstract the vertex and fragment processing pipelines into block-programmable functions, so you...