The calculator uses Naegele's rule: the due date is 280 days (40 weeks) after the first day of the last menstrual period. If your cycle is not 28 days long, the date shifts by the difference, because ovulation happens later or earlier. If you pick the conception date instead, the due date is 266 days (38 weeks) after conception. Only a small share of babies arrive exactly on the due date, so treat it as the middle of a window of a few weeks.
It is an estimate. Most births happen between 37 and 42 weeks. An early ultrasound can refine the date, and your doctor or midwife will confirm it.
The first trimester runs through week 13, the second from week 14 to week 27, and the third from week 28 until birth.
See also: Add Days to a Date · Age Calculator · BMI Calculator