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(えんぴつ)、木(き)、ボトル、指(ゆび)など、細長(ほそなが)いものを「一本(いっぽん)」数(かぞ)える助数詞(じょすうし)です。発音(はつおん)は「いっぽん」です。