Auto Layout dynamically calculates the size and position of all the views in your view hierarchy, based on constraints placed on those views - by Apple. Auto Layout can also be understood by describing it as a set of rules you use for each of the views in your view hierarchy, which rule defined how the alignment and shrinking or extending of the view will be performed according to different screen sizes of devices.

We should use Auto Layout in all of our projects because this helps us to manage the design of the screens for all the available devices we are targeting. For this we just need to understand what it is and how it perform and after we understood how it works, deigning dynamic layout for multiple devices will be fun.