I keep hearing this in the forums but I haven't found anyone explaining it. What does it mean when they talk about timing the electric engines and how does one do it??

Technically, they're electric motors, but that's beside the point.

Timing is only applicable to brushed electric motors, and for the most part, only to electric motors that have moveable endbells (the part on the end that holds the brushes). In a nutshell, the endbell is rotated in relation to the rest of the motor in order to modify the performance of the motor. Some people time them for best power, some for duration, and some for reverse rotation. It's a bit of a scientific process, requiring fixed voltage source and way to measure Amps.