What is {{$guid}} used for in Postman?

GUID is the acronym for “Globally Unique Identifier”. A GUID is mainly used to produce hexadecimal digits with groups separated by hyphens for uniqueness purposes, for example:

b3d27f9b-d21d-327c-164e-7fb6776f87b0

In postman you can use this to generate and send a random GUID to your api as required:

{
    "id": "{{$guid}}",
}

On Send would produce(with the random example above):

{
    "id": "b3d27f9b-d21d-327c-164e-7fb6776f87b0",
}