Labels are a feature that affects the process of signing and sending documents. It enables granting access to documents for selected people within a company. Thanks to it, specific people from the organisation, despite not being involved in the process of signing a given document, can access it.
Shared labels allow you to easily manage and work on documents, e.g. as part of a specific team in the company. Example? You manage the HR department and periodically send contracts for signing. You want your entire team to have access to them, such as being able to view them or give opinions. In this situation, you don't have to send the contract to the whole team each time. Simply create a shared label and add all the people who should be involved in the process. From now on, every contract will automatically go to the people you indicated.