I am using a Lenovo Ideapad Y480, and because of a corrupted dual boot, I am considering resetting the BIOS to "factory". However, what does this action do?
Resetting/modifiying a machines BIOS is not something that you should do on a whim. Although its not something that happens often, you can render your machine inoperable, even to the point where it can not be fixed. This does not happen often, but there is a small possibility it can happen.
Since you do not know what resetting the BIOS to factory settings does, I would highly recommend against it. If you are having an issue with the PC, try to fix it before attempting anything with the BIOS and talk to someone who knows about computers.
That being said, someone posted an answer that seems to relate to your issue specifically. Hopefully, your problem can be solved without having to modify the BIOS.
Now to answer your question about what resetting BIOS does. Most often, resetting the BIOS will reset the BIOS to the last saved configuration, or resets your BIOS to the BIOS version that shipped with the PC. Sometimes the latter can cause issues if settings were changed to take account for changes in hardware or OS after install.
This is what bios resetting does: http://pcsupport.about.com/od/fixtheproblem/tp/clearcmos.htm
From what I understand this will not help your dual boot problem. I believe you will need to fix your system on the hard drive level. You will need to simply install a boot loader like grub or windows depending on what operating system you are attempting to boot.