These numbers define an ellipse, orient it about the earth, and place the satellite on the ellipse at a particular time. In the Keplerian model, satellites orbit in an ellipse of constant shape and orientation. The real world is slightly more complex than the Keplerian model, and tracking programs compensate for this by introducing minor corrections to the Keplerian model. These corrections are known as perturbations. Drag becomes an optional eighth orbital element.
Orbital elements remain a mystery to most people.
To make matters worse, sometimes several different names are used to specify the same number. The following definitions are intended to be easy to understand. More rigorous definitions can be found in almost any book on the subject. A set of orbital elements is a snapshot, at a particular time, of the orbit of a satellite. Epoch is simply a number which specifies the time at which the snapshot was taken.