A restricted link is only valid for individuals who have been granted permissions to a resource.
For example, if the resource is shared with A, B, and C, and you send this link to D, D will not be able to access this resource.
This can be very helpful for when you are trying to avoid having an individual wait for an invitation to the resource, you can simply give them access, and send them the link to the resource in a personal email. It also is useful for sharing sensitive information, even if someone obtains a link to the resource, they will not be able to access it