The ability to set a custom URL is a premium HelloSign feature which allows you to redirect your signers to a web page of your choice after they've signed a document or template.
As team admin, you can lock this setting for your team members. This means that the URL you set will be the redirect URL for all documents sent from anyone on your team.
Or, you have the option of leaving the setting unlocked. If you do so, then each team member will have individual control of the redirect URL on any documents that come from him or her.
For help on how to set that up, please see:
How do I set a custom URL for my document?