Here's what that means:

* Sidereal period: The time it takes for Earth to complete one full orbit around the Sun relative to the stars.

* 365.256 days: This is the actual length of Earth's orbit, which is slightly longer than the 365-day calendar year we use.

Why the difference?

The difference between the sidereal period and the calendar year is because Earth is also rotating on its axis. While Earth is orbiting the Sun, it's also spinning. This rotation is what causes day and night.

To keep our calendar year in sync with the seasons, we have leap years. These extra days account for the extra 0.256 days in Earth's sidereal period.