A tab-delimited file is a text file that's set up with a tab separating each column of text. Mailchimp accepts these types of files for contact imports, and helps you map tabbed columns to a field in your audience.
Tips for setup
Before you export your file from a database application, take a look at their FAQs or support page for instructions on how to save as a tab-delimited file. It's a standard export format, but since every application is different, the steps will vary.
You can also export data as a tab-delimited file from most spreadsheet applications. Make sure your file ends with a ".txt" extension.
After you have a tab-delimited version of your audience, open it in a spreadsheet, like Microsoft Excel, to arrange and format your columns for import to Mailchimp. When you export the final, edited document from Excel, make sure to save it as a tab-delimited file again.