What does GPLv3+ mean?

Question

open source

Sometimes I see this line:

GPLv3+ License: GNU GPL version 3 or later gnu.org/licenses/gpl.html

There is a GPLv3 license, the link above just leads to its description, but there is nothing to say about GPLv3+ or its decryption. Let's say that GPLv3+ is the same GPLv3 or newer (as written above), but what does "newer" mean in this case?
Slightly exaggerating the question: is GPLv3+ a reserve to become in the future like GPLv3.1 or GPLv4? Or the first and second cases together? Or perhaps something completely different is meant here?
I just wish I could figure it out.

Answer 1

This means that if a new version of the GPL (v3.1 or v4) is released, it will be possible to freely relicense the source code already under it. And anyone can do it.
If the version is strictly limited by GPLv3, then only the author(s) can relicense the code. That is, the consent of all authors is required.
GNU recommends using GPLv3+.