Acceleration is when the nozzle starts moving. The nozzle must ramp from 0mm/s to the designated extrude speed (e.g., 40mm/s). This can make the first few mm of extrusion look different than the rest of the line. Normally not an issue, but in rare cases it might be. Jerk is the initial speed setting the motor controller is set to; it can be zero, but getting to some speed quickly helps reduce stringing. But too high a jerk will cause stalls (aka: layer shift). Once the controller is spinning the motor at the jerk speed, it ramps the speed up over time, so many mm per second. Actual distance traveled during this ramp period depends on many factors, but an estimate is possible by doing the math. Similar control is used for slowing the motor to avoid overruns.