The purpose of the Record Source property is to set the data source of the data for the form.
For example, if you would like to create a data entry form for the Suppliers table, you would specify Suppliers for the Record Source. If you add a new record in the form, it will insert a record into the Suppliers table. If you modify a record using this form, the data in the Suppliers table would also be modified.
You can set the Record Source property to a table name, a query name, or you can build your own SQL statement by invoking the Query Builder inside the Microsoft Access Database Window. You can invoke Query Builder by clicking on the button with the three dots to the right of the Record Source property.