I'm getting a "red minus icon" next to all my .h and .cpp inside of the VS 2017 solution explorer, anyone knows what does it means? It is red so it doesn't mean anything good, right?

Those sorts of symbols are generally indicative of version control states. A red minus beside a file usually means that file is excluded from version control. If you selected "add to source control" when creating your project, a default .gitignore would be generated, so take a look at that.

If it is the mark shown below, it means the file has been excluded from that build configuration.

It is extremely common in multi-platform development.  Right now on my other monitor I've got this as an example:

Right-click on the file for properties, you will probably find the "excluded from build" flag is set.  You can change the flag on a per-configuration and per-platform setting.  In this case XBox, PlayStation, and Switch each have their own files, but they're excluded from each other's builds.

There are other uses, such as having one file used for debug and another used for release. Either way, it is visual studio's way of showing you the file exists in the project but won't be compiled in with the current build settings.

I did set up SourceTree and ignored some folders from my UE Project as part of an UE tutorial, though how does Visual Studio knows about it? I did it all trough SourceTree.

Also, I did the procedure again on SourceTree with a new project, and now instead of the red minus, inside VS next to the file I have a blue padlock. So is this SourceTree's doing? I think the difference is that the second time I chose NOT to ignore the /Intermediate/ folder

There are many different overlay icons.  Several colors and styles of padlocks, colors of checkmarks, a variety of flags, various colored "no" signs like red and blue diagonal in a circle, red or blue x in a circle, red minus sign in a red circle, white - in a red circle, various pluses, exclamation marks, hourglasses, question marks, unavailable signs, and more.  Tools and add-ons can add their own overlays in addition to the ones that come standard.

I know of several blue padlock icons.  Each one means the file is not editable, but they can mean the file is marked as read-only, or that the file is in version control but not checked out, or that the file is locked within version control, or other things besides.

There are also a few other red minus sign icons. The ones I mentioned above were for items excluded from the build. There are similar icons meaning the file is not in version control among other things.

First, hover over the icon to see if there is some descriptive text. If that doesn't help, post a screen shot of the specific icon so hopefully we can identify it.

Here's an image:

I've also noticed there is a blue arrow inside a blue square. If I hover over one file, it says "archived" (or something like that, since my visual studio in not in english, so I'm translating here)

12 hours ago, MarcusAseth said:

how does Visual Studio knows about it? I did it all trough SourceTree.

Source control is typically stored in the directory with the code. You'll usually find a special directory, e.g.  .git or .hg, and that contains all the information about the repository.

The blue padlock looks like the Git plugin's source control status. That blue lock is the checked-in marker. The icon could be replaced with a red check if the file has changed, and a green plus if the file is new.  From this image, marker 1 shows the blue padlock meaning "checked in", marker 2 shows the red checkmark meaning "changed", and marker 3 shows the green plus meaning "new file":

I don't recognize the small blue box with an arrow that you show in your image.  It may be coming from another plugin.

Thanks frob, not it makes a little more sense