MobileBackups is using too much space
Mobile backups are a part of Time Machine called "snapshots" basically it lets you restore to one of the most recent backups without the need for you time machine disk. 

I believe OSX will automatically free up this space when it is need, but it can be turned off manually as well. 

See https://support.apple.com/en-au/HT204015
And http://osxdaily.com/2011/09/28/disable-time-machine-local-backups-in-mac-os-x-lion/