It's called optibay. Many do it when installing an SSD. Usually a 256gb SSD used for the OS/Apps and then place their HDD in the optibay drive and just use it as a second drive where they store all their files.
Put a hard drive (or an SSD) into it. If you are going SSD, it is better to put solid state in the optical because otherwise the drive will spin up a lot.
Then you get extra storage space in it. To put a drive in there, its best to get a holder for it, which bundles the correct SATA adapter. You can find them on amazon pretty cheap ($10 maybe)