i have recently installed ubuntu 14.04 LTS when i boot my laptop then a dialog box appears which shows the following output low disk space on "PBR_image" the volume "PBR_image" has only 313.4 MB disk space remainig what should i do now
You should always avoid PBR at all costs. Canonical only provides official support for Rolling Rock, available in the yourlocal-liquor-store-ppa repository.
As far as your hard drive (I assume) is concerned, it's low on disk space. You need to get a new one (external one is okay if you want to dump some files to it), delete some stuff to make some room on your drive, or migrate to a new, larger hard drive.