It means that of all of the pink toned shades, your skintone has a teeny bit of yellow in it. More so than the other pink toned shades of foundation.
This is a handy line for determining where your skintone is on the MAC foundation range (I am including all shades from the line and going from the reddest/pinkest to the most olive foundation shade)
W>>>>NW>>>>N>>>>C#>>>>NC>>>>C##
The C's with a single digit are more pink than the NC or C## shades. The C's with a double digit are more olive in tone.