The sidereal year is the time for the Sun to return to the same position among the stars.
As the Sun and the stars cannot be seen at the same time, this requires a little explanation. If you look every dawn at the eastern sky, the last stars you see appearing are not always the same. In a week or so you notice an upward shift. So in July you can't see Orion at the dawn sky, but in August it begins to be visible. In a year, all the constellations give a full turn.
If you're on the habit of looking at the sky before dawn, this motion is much more noticeable and much easier to measure then the North-South shift of the sunrise point in the horizon, which defines the tropical year our calendar is based on. That's why many cultures started their years the first some very special star (Sirius, for instance) could be seen at the East at dawn. In the work of Hesiodus, "the labours and the days", the times of the year for sowing, harvest... are given by reference to the first visibility of stars.
Up to the time of Hipparcus, the years measurd by the stars were thought to be exactly as long as the tropical years.
In fact sidereal years are very slightly longer than tropical years.
The difference is due to precession of the equinoxes.
26000 sidereal years make 26001 tropical years.