The Sacred Forest Meadow is Saria's favorite place, and there she plays the Saria's Song all the day.
It's pretty difficult to get into the meadow, because of the Lost Woods near it. That's why it's called "Sacred". At the time when Adult Link is going to the meadow, Mido is blocking the path. If Link plays the Saria's Song, he let's Link pass.
After Ganondorf's "transformation" to the King of Evil, the moblins found the meadow and began to guard the place.