Filesystem maps to the physical hardware you use for your filesystems. In UNIX everything is more or less represented as a file, so your
/dev/sda1 is nothing more than your hard drive. You can use
udevinfo -q all -n /dev/sda1 to get more information about it if you're curious.
tempfs is a little different. It's a filesystem that is made from a block of your RAM, but besides that, the principle is the same.
The "mounted on" directory tells you where the files stored on the Filesystem devices are mapped to in your directory structure. So everything stored in /dev/sda1 is accessible from / while everything in tempfs is available in /dev/shm.
Since /tmp is mounted as part of / which is part of /dev/sda1, you have 39G free on the filesystem that /tmp is part of.
The best way to check for free space is to run
df directly on the directory you are curious about:
df -h /tmp