I never owned a Skip-It, but I remember friends that did. It's quite hard to describe but basically it was a ball on the end of a rope. One end of the rope had a loop in it that you put one foot through and then you had to swing the ball around you with your foot and jump over the rope as it came round, kind of like skipping.
Later Skip-It models had an 'odometer' on the ball so you could keep track of how long you kept the ball going without tripping on it. Skip-Its were a favourite toy to be banned from the playground, as once in 'the zone' it was very difficult to keep track of who was around you, resulting in a few ankle injuries. In fact, even if you were the Skipper the very hard plastic ankle strap would result in some serious bruising.