When something is confining it limits you and holds you in. If you find living in a small town confining, you probably feel claustrophobic, like there's not enough room there for you and your aspirations.
The -fin- in confining is the same root that's found in words such as the French word fin, meaning "end" or "limit." The con- is the familiar Latin suffix that means "together." So when something is confining, it's like a border drawn around you. In some cases, confining can be a good thing, like when your efforts at confining the fire to the kitchen saved the rest of the house.