origin is the default name of the remote git repository you cloned from. Have a look at .git/refs/remotes/origin/* and .git/config within your sources to see how git knows about it.
git has a concept of "remotes" - these are like easy nicknames for a repository, so you don't have to use its full URL every time you want to refer to another repository.
origin is just a remote like any other, but you see it very frequently since when you clone a repository for the first time, git clone will by default set up a remote called origin to refer to the URL that you cloned from.
If you do git remote -v that will show you all the remotes you have set up in your local repository, and the URLs that they refer to. (You'll see that it's a bit more complex than I said above, in that a remote can refer to a different URL for pushing and fetching, but you probably don't need to worry about that. :))
"Origin" is the name of the remote repository where you want to publish your commits. By convention, the default remote repository is called "origin," but you can work with several remotes (with different names) at the same time.
origin is remote created by the git itself when you for the first clone the repo to point the URL from which you created the clone.
eg:
origin git@github.com:/PROJECT_U
n Git, "origin" is a shorthand name for the remote repository that a project was originally cloned from. More precisely, it is used instead of that original repository's URL - and thereby makes referencing much easier.
Note that origin is by no means a "magical" name, but just a standard convention. Although it makes sense to leave this convention untouched, you could perfectly rename it without losing any functionality.
In the following example, the URL parameter to the "clone" command becomes the "origin" for the cloned local repository: