Variables are placeholders for data that will be only known at flow runtime. Variables let you use data from one component in other - connected - components. For example, a component that triggers for new rows in your spreadsheet passes row data to a component that sends an email for each new row. In the email body, you want to use the actual data of the new row, referenced by column names.
Most inspector input fields have a variables picker which you can open to insert variables.
In our example, we have the "Date" and "Price" variables which correspond with the column names of our spreadsheet rows.