Fine Uploader, GNU GPL 3, how to understand the license?

Question

JavaScript

There is such a thing fineuploader.com
GNU GPL 3 LICENSE , fineuploader.com/purchase_form.html
Now I'm thinking of attaching it to one project and I'm worried about possible problems, and in general it's interesting to figure it out for the future.
How does the GNU GPL 3 work in this case?
It is said to be free for non-commercial open source projects.
I don't know much about legal things and have a few questions:

1. What is a commercial project?
   In the future, monetization through advertising is possible in the project.
   
2. What does open source mean?
   After all, this thing can be considered as a module that I use for the project, I don’t change the source code, should I open the code of the backend part too?
   

Answer 1

How GPL3 works .
A commercial project is a project whose goal is to make a profit. If there are problems with the goal, then you need to look at the legislation. In your case, it is better to consider that it is commercial, I think so.
Here you can read about open source code. In your case, if the open part of the software is used as a module, then only this part can be opened. The rest is released by you under any license.