To break down the commands you are looking at:
bcdedit /export C:\BCD_Backup
** Export the current BCD (Boot Configuration Data) to C:\BCD_Backup
** Change current working drive to the C: drive
** change the current working directory to the hidden "boot" directory on the current drive (C:)
attrib bcd -s -h -r
** remove the SYSTEM, HIDDEN and READ-ONLY attributes from the file "bcd" (where boot configuration data is stored)
ren c:\boot\bcd bcd.old
** rename the "bcd" file to "bcd.old" (backing it up)
** actually rebuild the "bcd" file, from scratch
As noted on this Microsoft help page
The /RebuildBcd option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option lets you select the installations that you want to add to the BCD store. Use this option when you must completely rebuild the BCD.
Basically you are backing up then removing the Boot Configuration Data and then forcing a full rebuild of it.