String skipping is pretty self-explanatory. It is the method of moving from one string to another while skipping a string. It is a technique used usually for practical purposes. A guitarist might choose to string-skip in the same fretboard position instead of playing notes on noticeably different parts of the fretboard. Using string skipping allows a guitarist to play in the same area of the fretboard, while simultaneously playing notes that are very far apart in pitch.
In this exercise, we're going to play a riff in the key of A Major. Use only your index finger to tap for now. When string skipping, do your best to somewhat "look ahead" while you're playing. You should always be looking to the next note or string so you're prepared for whatever is coming next.