Satellites far away orbit slowly. Satellites in circular orbits travel at a constant speed. We just specify that speed, and we're done. Satellites in non-circular orbits move faster when they are closer to the earth, and slower when they are farther away. The common practice is to average the speed.
Common units are revolutions per day.
Now that we have the size, shape, and orientation of the orbit firmly established, the only thing left to do is specify where exactly the satellite is on this orbit ellipse at some particular time. Mean anomaly is simply an angle that marches uniformly in time from 0 to 360 degrees during one revolution. It is defined to be 0 degrees at perigee, hence is 180 degrees at apogee. This relation does hold for two important points on the orbit, however, no matter what the eccentricity. It has become common practice with radio amateur satellites to use Mean Anomaly to schedule satellite operations.