What power does Bilbo's ring give when worn?

The ring gives Bilbo invisibility when he wears it.

Bilbo finds the ring in the goblin tunnels beneath the Misty Mountains after becoming separated from Thorin’s company. He later puts it on while escaping Gollum and discovers that Gollum can no longer see him. This lets Bilbo follow Gollum toward the cave’s exit and evade the goblins.

The ring is later identified as the One Ring, although The Hobbit originally presents it more simply as a magical ring of invisibility. Tolkien revised later editions after developing the Ring’s larger role in The Lord of the Rings. In that expanded mythology, invisibility is only one effect: the Ring also extends a mortal wearer’s life, amplifies power and corrupts the will.

A common mix-up is treating invisibility as the Ring’s only power. It is the correct answer for Bilbo’s immediate experience in The Hobbit, but the Ring’s deeper significance involves Sauron, the other Rings of Power and domination.