I have read here and in other places in the web, spoke with a friend about this and decided that as a freshman (maybe more of an Ubuntu freshman then some here might think), I should map this command - I should make a map and thus learn it's whatabouts and whereabouts:
If for now you didn't understand something in this chapter - don't worry, it will get clearer as you will keep reading, but to understand this matter you do need to read on Environment variables (EVs), their values, and purpose. I will now try to explain the command in simple words and in a method of mapping, for newcomers like me, and only for newcomers. Tried to do my best here...
The original value of the EV "PATH" as it comes with Ubuntu 15.10, is:
In the command itself note we have two PATH phrases. The last one is $PATH - The $ says "print the original value or values of the EV next to you"; The EV next to it is the PATH EV.
We exported the path variable itself (made it available for sub processes as well, I.E processes that run in the CLI which are not actually the Bash shell, but run inside it (such as Drush, which is the Drupal CLI).
Besides the export we also expanded it: The first PATH phrase (PATH=) used us to add an extra value (~/.composer/vendor/bin:) to the original value (represented by $PATH).
The colon (:) in the end of the new value I mentioned in the above paragraph, uses to distinct the new value from the original one.
The "" are the area in which the value/s reside.
The ~ is the home folder.
I hope that after this mapping I did, the command will be clearer to freshmen like myself.