You probably want to look at
which will show you the remaining space on all your mount points. If you wanted to look at a single mount point, say /boot then you would use
just reports the free space in a human readable form using KB, MB & GB - if you don't use this option then it just shows you in KB. Its worth noting that if you use the df that is packaged with some other Unix systems it reports the free space in Posix standard 512-byte blocks, which can be really confusing when you expecting it to be in KBytes!!