The document discusses apses and apsidal distance in orbital mechanics. It defines apses as the two points in an orbit that are nearest to and farthest from the center of motion, forming the major axis of the orbit. The length of this major axis is called the apsidal distance. At an apse, the velocity of the orbiting body is perpendicular to the radius vector from the center of motion. The document then discusses using the (u,θ) equation to solve differential equations to find orbital paths given the central acceleration.