TMPFS is a filesystem that exists only in memory. When you reboot the Server, the content of the TMPFS is gone. This is perfect for mounting the /tmp directory

In your /etc/fstab

tmpfs /tmp tmpfs size=30% 0 0

The size option set the maximum size of the filesystem. In the setting above, it can take up to 30% of the total RAM. If you umount the file system, all memory is returned.