Data blending combines two separate data sources in one sheet. It's older than relationships, and still useful when the data can't live in one data model, for example a published data source and a spreadsheet of targets.
Connect the second source with Data > New Data Source. The first source you use in a sheet is the primary (a blue check mark in the Data pane); fields from the other are secondary (an orange check mark). Tableau links the two on fields with the same name, shown with a chain icon in the secondary source.
For each mark, Tableau aggregates the secondary data at the linking field and attaches it, like a left join from the primary at the level of the view. Secondary dimensions on the Marks card appear as ATTR(): one value per mark, or an asterisk when there's more than one.
When you can, relate the tables in one data source instead: it's simpler and handles more cases.