The specular term is just a single part of your lighting, usually combined with some ambient and diffuse lighting. It represents the reflected light and is usually computed based on the surface normal, light position and the view position - as opposed to the diffuse term that doesn't depend on the view position but only the normal and light position, and the ambient term that's just a constant everywhere.
The wikipedia article for Blinn-Phong shading (usually just "Phong shading") is probably a good place to start.