If we are not talking about transmitting video over CAT5 in which case the skew would be a completely different thing (the skew of the picture on the screen), the data networking skew-free CAT5 is "simply" a cable in which all pairs are almost precisely the same length. That means that the signal of a multi-pair application, such as Gigabit Ethernet, will arrive at the same time over each of the four pairs. It is a desired outcome but not easy to achieve because different length of pairs are caused by their different twist ratio (number of twists per foot) which is an important part of the cable's design.
Apparently some manufacturers are able to maintain pair separation even with almost same twist ratios by using either plastic inserts that keep pairs apart of by shielding or at least wrapping pairs individually.
Dmitri Abaimov, RCDD