In Salesforce a ‘callout’ is any https call that accesses an external URL i.e. some other webservice or website. A ‘callin’ would be the opposite i.e. some external web service or application using the Salesforce API to access Salesforce data
An Apex callout enables you to tightly integrate your Apex with an external service by making a call to an external Web service or sending a HTTP request from Apex code and then receiving the response. Apex provides integration with Web services that utilize SOAP and WSDL, or HTTP services (RESTful services).