On your TV remote, you probably have a button that lets you switch back and forth between two channels. Last App Switcher works in the same way for the two most recently used apps on your Android handset.
The app adds a little floating icon to your screen which you can place anywhere for ease of access. Tap this and you can switch to the last apps you used; it works just like pressing Alt-Tab in Windows. You can change the size and transparency level of the icon to make it less intrusive.
Last App Switcher is much faster than using Android's built in app switcher. You can choose whether your home screen should be included in switching if you want a quick way to access your home screen icons, or stick with just switching between apps.