Wither Skeleton Skulls are obtained by killing wither skeletons, which spawn in Nether fortresses. They have a 2.5% drop rate, which can be increased with the Looting enchantment, up to 5.5% with Looting III.
Wither Skeleton Skulls are primarily used to summon the Wither boss, requiring three skulls placed on soul sand or soul soil. They can also be worn as a helmet for decoration and placed on a note block to produce a wither skeleton sound when activated.