Skew is a propagation delay that is caused when a signal arrives at two places at different times. For instance, a signal is sent by a controlling device at time T0. A receiving device A acts upon the signal at time T1. A receiving device B acts upon the signal at time T2. If T1 is not equal to T2, the difference between T1 and T2 is the skew. The distance between devices and the cabling between your devices and signal paths within the devices themselves all affect signal arrival times.