This is a pile of automatic Time Machine backups waiting to be copied to an external hard drive.
If you don't use TM: go into System Preferences and switch TM off - you should regain your 90GB.
If you do use TM: plug in your designated TM external drive and wait for your backup to happen (be extremely patient, it's a lot of data!). You won't get your 90GB back for a week or so, but it will eventually be released.
If you want your 90GB back instantly, but do use TM, switch TM off and on again, the drive space should be returned but you'll have lost all that backed up data.