What it is
The date your pay clock started, adjusted for any breaks in service. Your years of service for the pay table are computed from this date, so this single field decides which column of the pay chart you are paid from.
What goes wrong
If PEBD is wrong, your Basic Pay is wrong forever, including every future raise. Prior service, delayed entry, and broken service are where it gets miscomputed. Verify it once against your records and you never have to think about it again.