A fragment shader is ran once for every pixel contained in-between your geometry's vertices. Its purpose is to color in your mesh so you can actually see what it is you are rendering.

To create a fragment shader, you must assign a vec4 RGBA value to an attribute called gl_FragColor.

Start by adding in the gl_FragColor attribute and assign it a vec4 with an R value of 1.0, a G value of 0, and B value of 0, and an A value of 1.0.