Take your pick and strum it in a downwards stroke over all of the strings. When you reach the other side, stroke the pick upwards over all of the strings until you reach the top. That is a sweep. The alternate would be to pick each individual string which is slower and gives a different sound.

String skipping is just playing a note on one string then skipping to another string for the next note. This can be tricky if the notes are far apart with one or more strings in between so it requires a different technique than just picking upwards and downwards on the same or adjacent strings.