This means that if you’re recording a video on your phone and you want to capture video from the front and back, maybe to provide a bit more context to your footage, then you’ll be able to switch on multi-cam where footage can be captured from the front and back at the same time. However, before you get too excited, it seems that the feature will be limited to certain iPhones.
This includes the iPhone X, XS, XS Max, and the XR, along with the 2018 iPad Pro. Also, there are limitations on the software, where you can only run one instance of multi-cam at the same time.
We can only assume that the reason why not all iPhones will benefit from this new feature is due to the hardware, where the newer iPhones benefit from newer and more powerful chipsets. iOS 13 is currently set for a release this coming fall where it is assumed that it will launch alongside the 2019 iPhones.