counter for poles, rods, etc.
Used as a suffix attached to numerals to count long, slender objects such as poles, fishing rods, bamboo poles, or flagpoles. Not a general counter for sticks or bars; limited to specific pole-like items.
I bought two fishing rods.
Three flagpoles are lined up.
The kanji 竿 originally means 'pole' or 'rod'. The reading かん is a Sino-Japanese reading used in this counter/suffix context. The exact historical derivation of this counter usage is uncertain, but it is a conventional extension of the noun meaning.