If you have a Kindle e-reader that's a few years old (or older), there's a good chance that it's going to stop working like it used to. Specifically, it could lose the ability to connect to the internet, making it harder or even impossible to download new content. And that's definitely a bummer.
The reason for this is that all popular mobile carriers are transitioning away from 2G and 3G networks to 4G and 5G. And older Kindle devices that don't support 4G and 5G, will lose their data connections. This isn't a huge deal for older Kindles that have built-in Wi-Fi, as they'll still be able to download new content over Wi-Fi, but older Kindles that are LTE-only — such as Kindles (1st- and 2nd-gen) and the Kindle DX (2nd-gen) — will basically become obsolete.
As for when these older Kindles will be affected, it depends on which mobile carrier that you have. Verizon is set to sunset support for 2G and 3G by the end of the year (December 31). AT&T is going to do the same by late February 2022. And T-Mobile by April 2022.
That said, it's not all doom and gloom for people with older Kindle devices. According to The Verge, you should've received an email Amazon this week that your device is going to lose some connectivity/abilities, but in that email it has also included a $50 discount code to purchase a newer Kindle — either a Kindle Paperwhite ($130) or a Kindle Oasis ($250) — as well as $15 worth of eBook credits.
So if you have an older Kindle e-reader, check your email. If you've received a notification from Amazon, you don't have to buy a new Kindle immediately — in fact, I'd suggest not doing so. You've got until December (at the earliest) to decide what to do and Amazon usually heavily discounts all its Kindles over the holiday period. So you might get an even better deal if you wait.