For a while now, the UK government has been asking various tech companies for access to the encrypted data of its users. Apple is the first to comply with this, although not exactly in the way the government intended. What Apple has decided to do here is just remove the option of encryption for UK users entirely. All messages, photos etc will be stored in a format that means that, in theory, anyone can see them.
The goverment were hoping that Apple (and other companies like Meta) would leave the encryption in place, but provide a "backdoor" so those encrypted messages can still be read by special request. However, as we have covered here in the past, there is no way to do this safely. If there's a way to unencrypt data, there is always the potential that someone with malicious intentions will find out how to do that.
In Apple's case, this was a fairly simple thing to do. Their encryption solution (which they refer to as Advanced Data Protection - ADP) is a feature that users can toggle on or off. This makes it straightforward to just remove the ability to turn it on in the UK. However, the government also want this sort of access to other services like Whatsapp and Signal. There is no existing "off switch" for encryption here, and it wouldn't necessarily be trivial to add one even if they wanted to. Meta have already suggested they would just turn off Whatsapp in the UK rather than complying with a request like this.
It remains to be seen how effective demanding access to data like this will be. It seems likely that anyone that is sending and storing things that it doesn't want the government to see will just use another service (and there will always be another service - the tech world is always moving much quicker than goverments can legislate around). This leaves the change pointless at best for the users remaining on the unencrypted service, and at threat of malicious access to their data at worst.
In terms of what's happening right now for Apple users, anyone in the UK attempting to turn on Advanced Data Protection on their Apple device will get a message saying that's no longer possible. Anyone that already has it enabled will find it disables itself in the coming weeks.