Romanticism has a few key features. There is typically a sense of carpe diem surrounding pieces of romantic literature. It is generally focused on emotions versus logic, which is a direct break from the previous period of the Age of Reason. Generally romanticism is focused on an individual's importance or place in the world. But probably more than anything else, a romantic poem is nature-centered. It will talk about the importance of nature, or the beauty of nature, or what nature can teach humans, etc. Often a poet will write about "communing" with nature in order to gain some kind of transcendent knowledge.
"To Autumn" most easily fits with the importance of nature characteristic. The poem consists of three stanzas about the importance and beauty of autumn. To the speaker though, autumn is more than just a season. It is a close friend . . . a person.
Season of mists and mellow fruitfulness,
Close bosom-friend of the maturing sun;
Throughout the poem autumn is doing stuff people do. It's being personified. It does things like "bend" and "sit."
To bend with apples the moss'd cottage-trees, . . .
Sometimes whoever seeks abroad may find
Thee sitting careless on a granary floor,
A reader can't help but feel relaxed and calm as they read the poem. Autumn is a time of relaxing because the harvest is done and the harshness of winter is still a ways off. That's why the poem is a typically romantic poem. Its focus is on the beauty of nature and the peace that it brings people.