When people refer to a particular piece of development aoftware as closed or open source, they are referring to the license/availability of that software's code. You can use proprietary software to produce open source code, which is the case with Android Studio. The code that makes up Android Studio is not open source, but your own work made within it can be.

In general, "open source" is a broad term that just means "can I see the code that made this?". There are differing degrees of open source software as well. The MIT license, for example, opens up code to some modification/re-use but protects some libraries. Something like a BSD or GPL license is far less restrictive, usually allowing free modification and use of the code. Android Studio falls under the Apache license, one of the more restrictive licenses that still applies copyright, and may employ proprietary libraries that cannot be modified or copied for use. Again, this ultimately isn't likely to affect your own work or projects, but it does mean there's less transparency about the tools you are using to make it.

I apologise if this is overwhelming, but the distinction is important, and I think that as a beginner it makes sense to start with where there is the most documentation and ease of entry. Once done, it's definitely easier to move towards projects that more closely align with FOSS philosophies.