1.
If a place is hemmed in by mountains or by other places, it is surrounded by them.
Manchester is hemmed in by greenbelt countryside and by housing and industrial areas...
PHRASAL VERB: usu passive, be V-ed P by n
2.
If someone is hemmed in or if someone hems them in, they are prevented from moving or changing, for example because they are surrounded by people or obstacles.
The company’s competitors complain that they are hemmed in by rigid legal contracts.
PHRASAL VERB: be V-ed P by n, also V n P