To explain the difference, I'll use the "B" and "High E" string as an example.
If you start the downstroke picking the "High E" then upstroke pick the "B" the pick remains inside the two strings, eg. "Inside Picking".
If you start the downstroke picking the "B" then upstroke pick the "High E" the pick goes to the outside of the two strings, eg. "Outside Picking".
Incorporating both techniques in some of your riffs will greatly help anyone struggling to control their Alt. Picking abilities.
Good Luck.