A filler is a guy/gal that you seemingly 'like' or have a 'crush' on. This person is made to fill in a feeling for a person that you really love but is not actually with you. It's for a lost sense of love- as if you need someone, anyone at all to love at all times.
Molly loves Bill.
Bill moves to New York.
Molly doesn't see Bill.
Molly's brain likes Tim. But Molly's heart loves Bill.