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.
本 is the general counter for long, cylindrical objects (pencils, bottles, trees, etc.). 竿 is much more specific, used only for pole-like items such as fishing rods or flagpoles.
one pole; one rod
broad daylight; high noon
top of a pole; pole tip
the highest state of enlightenment; the pinnacle of attainment
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.