Hephaestus gave it to him when he was a child after Apollo asked for it, needing the bow and arrows to protect his mother, Leto, from Python. After receiving them, Apollo cornered Python in the sacred cave at Delphi. Apollo killed the monster but had to be punished for it, since Python was a child of Gaea. Hence, he was exiled from Olympus for a long time.
The bow can cause health or famine, although its main function is that of a regular bow, but with much greater power. It is described as the opposite of Artemis' Silver Bow; where her bow kills painlessly, like the soft rays of the moon, his tears life away like the harsh rays of the sun. It is equal in power to Artemis' Bow.
Apollo uses his bow during his battle with Typhon.