What is the use of Sharing Set in Salesforce Community?

A sharing set gives community users access to records that are associated with their accounts or contacts based on their user profiles.

Note:
The Available Objects list excludes:

Objects with an organization-wide sharing setting of Public Read/Write
Custom objects that don’t have an account or contact lookup field

1. Go to Customer Portal Settings.

2. Click New in Sharing Sets section.

3. Give any label and name. Select the Profile, set up the objects and select the access level.