Change sets are to send customizations from one organization to another organization.
When you want to send customizations from your current organization to another organization, you create an outbound change set. Once you send the change set, the receiving organization sees it as an inbound change set.
Sending a change set between two organizations requires a deployment connection. Change sets can only be sent between organizations that are affiliated with a production organization—for example, a production organization and a sandbox, or two sandboxes created from the same organization.