Inbound means that other applications make calls to ServiceNow’s REST API to get information from SN, or create/update records. Outbound means that ServiceNow makes calls to other applications’ REST API to get information from the other application or create/update records.
Inbound web services are designed to provide third parties with the ability to retrieve (GET) or update (POST) data in ServiceNow, while outbound web services allow ServiceNow to initiate a transaction with a third party (also using either GET or POST, etc.)