That's one definition. Here's another:
Let's assume that you've got several email addresses, and that you use an email client like Outlook Express. When you click 'New Mail', the address that you'll send through (and which will be shown when your mail gets to the recipient) needs to be entered in the 'From' field. Outlook Express automatically fills in your 'default' email address. (If you want to change the account which you'll send through, simply click in the 'From' box, or on the little arrow next to it, and select an alternative address).
To change the default address in Outlook Express (so that another of your addresses is automatically filled in the 'From' field), go to Tools > Accounts > Mail. Click on the account you want to use, and then on 'Set as default'.
Chris