cp /home/username/text.txt cp /home/username/text2.txt
there cp is copy., /home/username/text.txt is argument 1 and /home/username/text2.txt argument 2.

that is copy 1 to 2, not much more than that.

even a regular expression could be an argument, say cp -a /home/usernamne/* /home/usernamebackup/ the -a is preserve permisions and be recursive (for more look in man cp),
that would copy everything including dirs subdirs with permision preserved... from /home/username to the dir /home/usernamebackup.
and as you know /home/usernamebackup can be a different disk temporarily mounted there, or a diferent partition/disk in the computer...
these things will becoe more clear after a time of use.