It's an email program. It will bring your email to your computer, so you don't have to log in online at a website. You can also save your emails in the program on your own computer instead of only online. Or you can save emails both ways..depending on how you set it up.
more info-
https://www.mozilla.org/en-US/thunderbird/

Top 11 Free Email Programs for Windows
http://email.about.com/od/windowsema...email_prog.htm