As you are completely new:
A web page can reference content from another site by passing the full url, eg:
<img src="http://www.someotherplace.com/mystuff/mypicture.jpg">
The part where this gets tricky is the "seamless" bit. The above is seamless for the website visitor, but for the website editor (you) it is more fiddly than using ’onsite’ files, because you need to know exactly where the file is.
Whilst you can navigate within MODx’s image browser to images on your MODx server, it isn’t able to list the files you have elsewhere. Many MODx galleries are dependent upon being able to list the local directory.
Now, I don’t know anything about Amazon’s S3 storage bank so I don’t know whether there are any tools or facilities to make this easier.
I know the issue of listing offsite directories has come up at least once but I’m unaware of any MODx solution.
However, the basic answer is that yes you can use offsite content, it’s just not as easy to select and manage as onsite content.