RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Nitroglycerin
The Old School RuneScape Wiki also has an article on Nitroglycerin.

Nitroglycerin is a members-only quest item. It is used in the Digsite quest to make an Explosive compound along with Ammonium Nitrate, Ground charcoal and Arcenia root to blow up bricks blocking the path below the digsite.

Dropping this item will result in the player taking damage.