Ascending & Descending Nodes

        The Ascending Node is when an object moves north through the plane of reference.   Example when orbiting Kerbin when your craft crosses the equator heading north that exact moment is the ascending node.  

        The Descending Node is the southern movement for the same thing        

These are also the points where you want to burn to change your inclination to either make your orbit more polar or more equatorial.

        When Matching orbits though you need to match inclinations with the other object.   Now your ascending/descending nodes are based on the other objects orbit and you want to burn at one point or the other to get the two orbits to be on the same plane.