If you say Y, then the VESA standard, Generalized Timing Formula
or GTF, will be used to calculate the required video timing values
per video mode. Since the GTF allows nondiscrete timings
(nondiscrete being a range of values as opposed to discrete being a
set of values), you'll be able to use any combination of horizontal
and vertical resolutions, and vertical refresh rates without having
to specify your own timing parameters. This is especially useful
to maximize the performance of an aging display, or if you just
have a display with nonstandard dimensions. A VESA compliant
monitor is recommended, but can still work with non-compliant ones.
If you need or want this, then select this option. The timings may
not be compliant with Intel's recommended values. Use at your own
risk.

If you say N, the driver will revert to discrete video timings
using a set recommended by Intel in their documentation.

If unsure, say N.