The mouth of something can mean the place where a river flows into the sea or a larger body of water. For example:
We‘ve got to get our hero,
to the mouth of the Nung River
so he can go pursue Colonel Kurtz.
It was here
that Homer described the Greek encampment
at the mouth of the Scamander River.
Located at the mouth of the Bay of Bengal,
this coastal village was built on top of the Meghna River delta.
It can also mean the opening of a cave or other underground cavity. For example,
To give you an idea of scale,
this is looking down towards the mouth of the Beardmore Glacier.