Textures are made up of 3 or 4 channels. R (red), G (green), B (blue), and optional A (alpha).
In a material you multiply a colour or a texture by each on of these channels and then mix the results together to create a whole new texture that is then applied. In this case the red channel is a mask of lines, the green is a mask of squares etc.
Basically you can use the channels of a texture as black and white masks in your material to modify things another example could be red=metallic and green=roughness. You would connect red to the metallic port and green to roughness. In photoshop you would then paint just in this channels. In red you would paint white for metallic and black for non metallic. In green you would paint white for rough and black for smooth.
The easiest way to understand this is open the the texture I a paint program and look at the channels. Then in the tutorial I assume they show the material being edited. Pause it and see how the lines join up.