Estimated due date from your last menstrual period or conception date (Naegele's rule).
An estimate only — only about 5% of babies arrive exactly on the calculated date. Your doctor's dating scan is more accurate.
"From last period" adds 280 days to your LMP. "From conception date" adds 266 days, since conception is assumed to occur about 14 days after the LMP. Both modes land on the same due date for a standard 28-day cycle.
It's the standard convention of estimating a 40-week (280-day) pregnancy counted from the LMP. It assumes ovulation around day 14 of a 28-day cycle, so it's a population-average estimate rather than a precise prediction for every cycle length.
As the note below the result says, only about 5% of babies are born exactly on the calculated date — it's a statistical estimate. A doctor's dating ultrasound, based on actual fetal measurements, is generally more accurate than date-based calculations alone.