Apple's iPhone and iPad support several options to display on larger screens, including using a VGA, HDMI, or DVI adapter. In addition, Apple supports its Wi-Fi-based AirPlay mirroring technology that allows streaming of the iPhone or iPad's screen to supported devices. This allows for viewing through projectors and on televisions which have this technology.
While this feature is convenient if you have a device that supports AirPlay mirroring, you might find that Apple's own line of Macs lack this feature. As with iOS, Apple has included AirPlay mirroring in OS X to stream to devices that support this feature, but has not provided a way to stream from one system to the next.
Granted, Apple supports screen sharing between Macs, and options to take screenshots in iOS for sharing on a Mac; however, if you would like to view your iPhone or iPad's output directly on a Mac, then Apple does not provide a built-in solution for this.
However, this ability is possible using third-party applications which implement an AirPlay server, which supports device mirroring. There are a number out there, with a couple of popular options being AirServer, and Reflector.
To use these apps, you simply launch them and with minimal configuration to ensure the best quality for your specific iOS device, you can then connect your iOS device via AirPlay from the Control Center panel.
These apps are relatively cheap at around $10, and come with a limited demo in which you can use them at full or near-full functionality. In addition to viewing iOS content on your Mac's display, you can use some of these apps to record your actions, which make for a quick way to create a short tutorial.