Pregnancy can't happen on just any day of the month. The fertile window exists because of two time limits. Sperm can survive in the female reproductive tract for up to about five days. The egg survives only 12–24 hours after ovulation. Put together, that gives roughly six fertile days per cycle — the five days leading up to ovulation, and ovulation day itself.
Using it
The highest-chance days are the two or three days immediately before and including ovulation. Sex on these days means sperm are already waiting when the egg is released. Ovulation kits (which detect the LH surge, the hormone rise that triggers ovulation), cervical mucus changes, and cycle tracking all help find the window. Or simply have sex every 2–3 days through the middle of the cycle. That covers the window without strict timing.