What is inbound and outbound changesets in Salesforce?

An inbound change set is a change set that has been sent from another Salesforce org to the org you are logged in to. A change set must be deployed for the changes to take effect. You can deploy the contents of an inbound change set as a whole but not on a component-by-component basis.
An outbound change set is a change set created in the Salesforce org in which you are logged in and that you want to send to another org. You typically use an outbound change set for customizations created and tested in a sandbox and that are then sent to a production org.