The Hobbit is an adventure story involving Bilbo Baggins, a Hobbit, Gandalf, a wizard and several other creatures including dwarves, elves, and a dragon.
This book is the precursor and introduction to Middle Earth where The Lord of the Rings trilogy takes place.
They are (or were) a little people, about half our height, and smaller than the bearded Dwarves. Hobbits have no beards….They are inclined to be fat in the stomach; they dress in bright colours (chiefly green and yellow); wear no shoes, because their feet grow natural leathery soles and thick warm brown hair like the stuff on their heads (which is curly); have long clever brown fingers, good-natured faces, and laugh deep fruity laughs (especially after dinner, which they have twice a day when they can get it). (The Hobbit, prologue)
A short summary is that Gandalf, the wizard proposes an adventure to Bilbo. Bilbo is a "stay-at-home" Hobbit and refuses to have anything to do with adventures.
Gandalf scratches a secret sign on Bilbo's door.
Shortly, a troop of 13 dwarves arrives at Bilbo's house with a treasure map. The map piques Bilbo's curiosity, and he is off on an adventure in search of the evil dragon Smaug who has stolen the Dwarve's treasure.
On this adventure they are accosted by trolls and orcs. In the caves of the Misty Mountains, Bilbo meets Gollum and acquires a ring.
A great battle ensues in which the dragon is deafeated, the dwarves regain their treasure. Thorin Oakenshield accuses Bilbo Baggins of treachery, later forgives him and commends Bilbo's bravery and courage. Thorin dies in this novel.
The dragon is defeated, and they return home. Bilbo returns to the Shire and settles in for a nice retirement.