Uncle Tom's Cabin is told from the perspective of an omniscient (all-knowing) third-person narrator. Using this narrative form, the narrator is able to get inside the minds of various characters and explain to us what they are thinking, feeling, and perceiving. This also keeps the plot exciting by moving from one story line to another, especially at the beginning of the novel. An omniscient narrator can move around geographically at will, moving from one place to another
The omniscient narrator thus moves, for example, from Mr. Shelby dealing with slave traders to Mrs. Shelby thoughts and anguish that her husband is selling slaves to Eliza's thoughts as she realizes her beloved four-year-old son is going to be sold on the auction block.
The omniscient narrator allows us to track Eliza's dangerous progress fleeing the Shelby farm with her son to dramatically cross the ice on the Ohio River to get to freedom. It allows us the visit the Quaker safe house where she stays before she arrives there. It moves to Tom in his cabin with his family and friends. It also allows Stowe to express scathing irony about the "kindness" of the slave traders, who think trinkets will compensate mothers for having their children ripped away from them.
It is worth noting, however, that much of the story is told through Tom's eyes as he is sold from owner to owner.