An URI (Universal Resource Locator) specify an internet resource, followed by optional parameters.
The first parameter is identified by a ?, the following ones by a &.
. If the parameters contain a ? or a &, they are encoded as %3F and %26. This is called Percent (%) encoding.
.https://fr.wikipedia.org/wiki/Percent-encoding

Exemple:

$ curl 'http://localhost:8001/register.php?name=jean&lastname=bon&age=42&admin=true'

As unix shell uses the & to launch background tasks, it is imperative to put the URL between quotes '.