Uncle Tom's Cabin promotes the idea that slavery is a terrible evil that needs to be abolished.
Stowe witnessed slavery firsthand, as she lived across the Ohio River from Kentucky, a slave state, but like other white people, she thought it would wither on the vine. However, when the Supreme Court upheld laws that harshly criminalized aiding and abetting escaping slaves, even in free states, Stowe felt this was a step backward and was galvanized to write her novel. Its goal from the start was polemic: it was meant to make a political point and drive people to action.
Stowe's novel shows that slavery is a terrible institution in any scenario and then describes Uncle Tom's situation gradually deteriorating as he passes into increasingly worse circumstances. For example, Tom is treated "well" on the Shelby farm, but even here he is at the whim of being considered a piece of property, like a sofa or a chair, when he is sold away from his wife and children to settle his owner's debts. Tom, after a series of changing hands, ends up in the worst of situations, beaten to death by a sociopathic master on an isolated plantation.
Stowe wrote the novel to tug at the heartstrings of white people by showing the plight of slaves from their point of view. She systematically takes apart common arguments in support of slavery, such as by showing that even "good" masters have the power to do terrible things and consistently illustrating that slaves are not "happy" with their lot in life.