Along that same one bend in the road from the story for 右 (right), there's a large stone blocking the entrance.
It might be beneficial to remember that this one stone is above the bend in the road. This will help differentiate this kanji from 右 (right).
Also, this does NOT mean boulder! Boulder has its own kanji (岩). This is just a large stone.
石 - Stone
JLPT Level: N3
Readings: セキ, いし