A query activity is an automation studio activity that allows querying data from data extensions or data views using SQL statements. A data extension entry source is a journey builder entry source that allows injecting contacts into a journey based on a data extension. Journey activities are actions that occur within a journey, such as sending an email or updating a contact attribute. By using a query activity to filter contacts who have not made a purchase in the last six months from their order data extension, NTO can create a sendable data extension with those contacts and use it as a data extension entry source for their journey. Then, by using journey activities, they can communicate the samemessage across all channels to those contacts. References:
https://help.salesforce.com/articleView?id=sf.mc_as_sql_query_activity.htm&type=5
https://help.salesforce.com/articleView?id=sf.mc_jb_data_extension_entry_source.htm&type=5https://help.sales