At Google I/O today one of the projects becoming official is Android Instant Apps. This basically means that you can run an app without installing it.
Behind that simple use case are some slightly more complicated tweaks that developers need to make to their apps for them to support this. That's because an app needs to be modular, so to speak, to be able to achieve this.
When, for example, someone sends you a link to a product listing on a website, you'll get to view it in that retailer's app - even if you don't already have it installed on your phone or tablet. Android will load only the relevant parts of the app on your device, on the fly, which means there won't be a lot of waiting to do while that happens.