tl;dr: As early as September 9th, some users accessing the site from old Android devices (running Android 7.0 and earlier) may see SSL warnings in their browser regarding an invalid certificate when they attempt to access the site.
The technology that allows Stack Exchange to serve traffic to users securely is called "SSL." To serve traffic via SSL, Stack Exchange needs to be issued a “security certificate” from a trusted provider. Our certificate is issued by Let’s Encrypt, one of the largest and most trusted security certificate providers on the web.
Without getting into technical details (which you may read on Let’s Encrypt’s site), Let’s Encrypt is making a change to how they operate their security certificates. This change will cause certain older devices, including most Android devices issued 2017 and earlier, to no longer trust traffic using Let’s Encrypt’s certificates.
Because Stack Exchange uses Let’s Encrypt, as early as September 9th, we expect that some users of Stack Exchange will see SSL warnings about an invalid certificate when they attempt to access the site. We estimate that 0.07% of traffic to the site will be affected by this change.
If you believe you will be affected by the change, or want to understand the technical details of the change, we encourage you to read Let’s Encrypt’s post on the topic. They offer an explanation as to what is changing and how users may be able to work around the issue.