Swing Documentation:
Many projects do not have notes positioned on a perfectly even time grid. For example, projects with a swing feel, though they may be written entirely in eighth notes, are often played more like eighth-note triplets, with the first note extended and the second one shortened. The Swing option lets you distort the timing grid so each pair of notes is spaced unevenly, giving the quantized material a swing feel.
A swing value of 50 percent (the default) means that the grid points are spaced evenly.
RobertWS's post in Did I miss something about "Quantize?" was marked as the answer