While in the air, holding A while turning left or holding D while turning right is considered a strafe. Each time you switch between left and right you start a new strafe.
Gain
Describes gaining speed. You gain speed when strafing properly.
Loss
Descibes losing speed. You can lose speed if you are not strafing properly. This is usually due to poor sync, but can also be caused by moving the mouse too fast.
Sync
How good a player is at syncing their keyboard to their mouse. Poor sync happens when you do not hold A while turning left and vice versa. This often happens for a fraction of a second between strafes. Doing this will cause you to have dead airtime (no speed) or even lose speed.
SSJ
Speed Sixth Jump; a measure of the speed a player at their sixth jump. Used to determine how good a player is at gaining speed.