Google's SMS app, Messenger, is one of those apps that does what it says on the tin: it sends and receives SMS messages, and can manage them too. With version 1.8.266, a new feature is coming to Messenger: the ability to manage messages which are saved to the SIM card. It's not clear if the previous versions of Messenger could see these messages but had no interface for displaying them, or if they were completely invisible.
A bit of a history lesson: back when phones had very little storage, the SIM card would sometimes be used for saving messages instead of the phone. This was so that when a user moved across to a new device, they could take any messages they'd saved on the SIM card with them (think texts that were special to them or had significant meaning). Not many messages can be stored on a SIM card - the number differs depending on the amount of storage on an individual SIM, but it's generally less than 50.
Left: previous version. Right: version 1.8.266.
Messenger's tool for managing SIM card messages is barebones. It will show any messages currently on the SIM card and allow them to be deleted. That's it. In the future, maybe this will get a function to copy a message to the phone, but currently that isn't an option. Similarly, there is currently no ability to copy a message to the SIM card - I had to use a third party app in order to test this out.
Left: the screen for managing SIM card messages. Right: a SIM card message selected, showing the 'Delete' icon.
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.