A fallback text is a text that mail merge adds to your mail when it can't find a value for a recipient in the Google sheet.
How fallback for mail merge works
Once you've selected a mail merge variable, you can define a fallback text by manually adding it after the variable tag.
For example, if you use the variable {{name}}:
You can add a pipe character i.e. | and then add the fallback text after it:
So, afterward, when you send the campaign, recipients with no value in the variable column in your Google Sheet will receive a mail with the fallback text instead of a blank space.
A recipient with Charles as the {{name}} value, for instance, will receive an email with the following greeting:
Whereas a recipient that doesn't have a {{name}} value in your sheet will be greeted like this:
If there's no value for a recipient in the spreadsheet and you don't define a fallback a blank space will be added to the mail instead.