A clipper guard is a plastic comb that clips onto the blade. Its number is the length of hair it leaves: a 1 leaves 3 mm, a 2 leaves 6 mm and an 8 leaves 25 mm.
For a fade the barber works up the head one guard at a time, from skin or a 0 at the nape to the longest guard or scissors on top.
Once you have a cut you like, ask your barber which guards they used. Next time, say the numbers and anyone at the shop can give you the same cut.