This is a counter for "one long, slender object." It is commonly used for items such as bottles, pens, trees, sticks, or fingers. Note the pronunciation change from "ほん" to "ぽん" for one.

鉛筆、木、ボトル、指など、細長いものを「一本」数える助数詞です。発音は「いっぽん」です。