Explainshell.com rocks! Nevertheless, you lose time to leave the terminal (open browser, copy-paste). But there is a cool solution from ManKier. All what you need is curl.

Usage

# curl request for whoami
$ curl -Gs "https://www.mankier.com/api/explain/?cols="$(tput cols) --data-urlencode "q=whoami"

# curl request for df -h
$ curl -Gs "https://www.mankier.com/api/explain/?cols="$(tput cols) --data-urlencode "q=df -h"

Simpler usage

With a tiny script it will be more comfortable! Add the following to your .bashrc or .bash_profile (MAC OS X).

# explain.sh begins
explain () {
	if [ "$#" -eq 0 ]; then
		while read  -p "Command: " cmd; do
			curl -Gs "https://www.mankier.com/api/explain/?cols="$(tput cols) --data-urlencode "q=$cmd"
		done
		echo "Bye!"
	elif [ "$#" -eq 1 ]; then
		curl -Gs "https://www.mankier.com/api/explain/?cols="$(tput cols) --data-urlencode "q=$1"
	else
		echo "Usage"
		echo "explain                  interactive mode."
		echo "explain 'cmd -o | ...'   one quoted command to explain it."
	fi
}

Now you can do …

# explain one command
$ explain 'df -h'
...
df(1)
  df displays the amount of disk space available on the file system containing each file name
  argument. If no file name is given, the space available on all currently mounted file systems is
  shown. Disk space is shown in 1K blocks by default, unless the environment variable
  POSIXLY_CORRECT is set, in which case 512-byte blocks are used. If an argument is the absolute
  file name of a disk device node containing a mounted file system, df shows the space available on
  that file system rather than on the file system containing the device node. This version of df
  cannot show the space available on unmounted file systems, because on most kinds of systems doing
  so requires very nonportable intimate knowledge of file system structures.

  -h (-H, --HUMAN-READABLE)
    print sizes in powers of 1024 (e.g., 1023M)

… if you insert only “explain” an interactive mode will started!