Backed up HD but no space?

Hey all, sorry if this is the wrong place for this question. I was low on space on my Macintosh HD so I backed up and moved all my music onto an external HD. However, when I open the "About my Mac" tab and check the storage on Macintosh HD it says there is about 100 ish gb of "backup" and I don't have any more free space than before! How do I get this to go away and free that space back up?