一 (yī): The number one; a single.
箭 (jiàn): An arrow. This character is composed of 竹 (zhú - bamboo) on top, representing the arrow shaft, and a phonetic component below.
双 (shuāng): A pair, two, double. The character shows two birds (隹) being held by a hand (又).
雕 (diāo): An eagle or a large bird of prey.
The characters combine literally to mean “one arrow, a pair of eagles,” creating a vivid and memorable image that directly illustrates the idiom's figurative meaning of achieving two significant things at once.