We don't necessarily describe everything it can do but rather some useful capabilities. The df command is used to show the amount of disk space that is free on file systems. In the examples, df is first called with no arguments. This default action is to display used and free file space in blocks. In this particular case, th block size is 1024 bytes as is indicated in the output.
Subsequent columns show total space, blocks allocated and blocks available.
The capacity column indicates the amount used as a percentage of total file system capacity. The final column show the mount point of the file system. This is the directory where the file system is mounted within the file system tree. Other file systems can be mounted in any directory of a previously mounted file system. In the second example, df is invoked with the -i option.