The frame perch is designed to hold three frames on the side of the hive. When you are inspecting your hive, you don't necessarily want to put your frames down into the dirt or the grass, which can make the honey and nectar on a frame dirty and cause the bees unneeded grief to clean it out. With the frame perch, you can simply set your frames aside, both upright and away from dirt and grass. It is also useful when you need to inspect multiple frames at once because it eliminates the inconvenience of having to hold both frames.
You can purchase a frame perch here.
Here is a useful video of Stan Moulton explaining how to use the frame perch: