String skipping means jumping over a string between two picked notes. It opens up wide, interesting intervals but takes accurate picking to avoid hitting the string in between.
The first two exercises use A minor pentatonic at the 5th fret. The first pairs each note with the same note an octave higher, two strings up: index on the low note, ring or pinky on the high one. The second walks through box 1, skipping a string every time. The third picks the Am and C chord shapes, skipping a string on most moves.
Keep strict alternate picking: down on the low note, up on the high one. Make the jump with a small, curved motion that clears the skipped string, rather than dragging the pick across it.
Mute the skipped string. The fretting finger on the lower note leans lightly on the string above it, and the picking hand palm takes care of the low strings.
The most common mistake is a bigger, wilder picking motion on the skip, which wrecks the timing. The jump should take exactly as long as any other note. Slow it down until it does.