iOS Multitasking on the iPad allows users to operate in a "split-screen" mode with other apps. Both apps, ForeFlight, and your other app of choice are required to support multitasking. If the other app does not support multi-tasking, then ForeFlight will not work in the "split-screen" mode with the other app.
You can learn more about ForeFlight's multitasking functionality by watching this video:
Video: Multitasking Support & Major Design Improvements
Note: Multitasking with ForeFlight is not supported on the iPhone.