A compost sifter is not a fancy piece of equipment. It is a simple wooden frame with some hardware cloth attached to one side. The hardware cloth acts like a large screen to catch plants, leaves, acorns, and sticks. But, the plain dirt can be broken up small enough to pass through the screen.
I use mine with a wheelbarrow to catch the dirt that falls through the screen. I can then take all the dirt and reuse it in the garden. Tomatoes often like a little extra dirt added to their base as they grow. But I can also use this dirt around the potatoes in a process called "hilling". This practice increases the yield from a potato plant.