"/app (/dev/dsk/c0d1s0 ):51430396 blocks 3098364 files" What is the size of UFS in the command above; does block means the total space allocation?
From the df man page:
/export/home (/dev/dsk/c0t0d0s7 ): 434364 blocks 108220 files where the columns represent the mount point, device (or "filesystem", according to df -k), free blocks, and free files, respectively.
As, chris specified, the -h flag will output the above in "(h)uman readable format." Other options that might be helpful are:
-b Prints the total number of kilobytes free -k Prints the allocation in kbytes.
For more information, reference the df man page.
$ man df
In general, man command, will return information on the command you are executing, and how to interpret its output.
df -h will tell you what you want
The disk is arranged in units called "physical blocks". Physical block size for UFS is by default 512 bytes, so 51430396 free blocks is equal to 25715198 kilobytes free.
(Depending on the OS version, sometimes the -h and -k parameters aren't available so its handy to know the meaning of the output.)