It’s called a path variable:
Path variables enable us to add dynamic routes.
See below example taken from the REST API handbook:
// Here we are registering our route for single products. The (?P<id>[\d]+) is
// our path variable for the ID, which, in this example, can only be some form
// of positive number.
register_rest_route( 'my-shop/v1', '/products/(?P<id>[\d]+)', array(
...
'callback' => 'prefix_get_product',
) );
The important part to note is that in the second route we register
(the one above), we add on a path variable/(?P<id>[\d]+)to our
resource path/products. The path variable is a regular expression.
In this case it uses[\d]+to signify that should be any numerical
character at least once. If you are using numeric IDs for your
resources, then this is a great example of how to use a path variable.
When using path variables, we now have to be careful around what can
be matched as it is user input.
And the format is: ?P<{name}>{regex pattern}.
And based on the above example, one could go to http://example.com/wp-json/my-shop/v1/products/123 to retrieve a single resource.
PS: I edited mainly to add the second paragraph above.