Alias means a second name for an existing thing. Aliases for command means a shorter command for an actual lengthy command. This will help in remembering it. For example, the command to untar/unzip a compressed file is:
tar xzvf [filename]
This is ofcourse not easy to remember and this is where aliases comes into play.
In the home directory of every user, there is a hidden file named .bashrc which can be used to define aliases for that user.