BASH has the ability to execute a command string and replace that string with the output of the command. Or to say it another way, the output of a command will replace the command itself.
There are 2 ways to accomplish this.
The first is to surround the command with backticks or blockquotes.
`command`
You can also substitute with dollar symbol and parenthesis.
$(command)
The following will do a direct substitution of the current username on the command line.
$ echo /home/$(whoami)
/home/ryan
Another great use is the ability to assign a command to an environment variable.
$ MYDATE=`date`
or using the other format
$ MYDATE=$(date)