If you do want to find out, there is a nice post from a couple of years ago by @griffin which mentions a way to check, as well as describing the general context.
The official explanation from Let's Encrypt is here:
As @aarongable says, if you didn't know this was a thing before, it's not likely that the change will affect you.
Here is Cloudflare's official discussion of the change:
The main potential impact they highlight is that about 3% of Android devices on the Internet may no longer be able to access sites that use Let's Encrypt certificates, unless the owners of those devices individually take additional steps (or replace them with newer devices!).