Progressive Web Apps (PWAs) are websites that allow you to install themselves locally on your device(s) so that they are accessible when there is no connectivity or your device is in airplane mode.
If you open a website that is a PWA on the chrome web browser you will be prompted to install it on your desktop or you can install it by clicking on the install icon in the address bar in the top right.
If you open a website that is a PWA on the Safari web browser you can install it by clicking on the share icon, scrolling down a bit and selecting add to home screen.
There is other content on this website but no direct links are provided here.