A His Dark Materials Primer:
His Dark Materials takes place in a world much like our own, with one significant difference - every person is born with a daemon, a physical representation of their soul in the shape of an animal. Children’s daemons can shift and change between shapes - and do often - and generally continue to do so until the child reaches puberty. At this point, the daemon “settles,” choosing the animal it will be for the rest of it’s life. The chosen animal - what they are, what they represent - reflects the human to whom the daemon belongs.
If a person dies, their daemon dies with them. A person can survive without their daemon (though the loss often kills them) except they will be a shell of themselves, lacking personality, emotions, and the ability to connect with others.