In ZBrush, the appearance of any surface is affected by several things - its base color, its texture image (if it has one), the lighting that falls on the surface, and its material. The material changes the way the lighting reacts with the surface so that it may appear - for example - shiny, bumpy, reflective, metallic or transparent. There are many preset materials in ZBrush to give you control over a scene. In addition, each material can be modified to create new materials.
What if you could create a surface material and lighting environment simply by painting an object so that it looks like you want it to look? What if you could create that surface material by sampling points from an existing image that already features the material and lighting environment you want? For a great many (probably the majority) materials and lighting environments, this is exactly what MatCap (Material Capture), allows you to do.