Lumps of concrete stuck in the ground usually mean you had fence posts installed at one time. Many fences are constructed with metal posts that secure to the ground with concrete footings or postholes. It is also possible that at some point a special project was constructed that required the setting of a concrete footing. If you plan to excavate or complete additional landscaping, often you must remove these lumps of concrete from the ground.
Excavate the ground around the concrete lump with a shovel. This will loosen the concrete in the ground and give you the ability to reach around it. Dig the area around the concrete large enough for you to easily reach in or large enough that you can roll the concrete out of the hole.
Grip the sides of the concrete with your hands and lift it out of the ground. If the concrete is too large for a single person to lift, use the help of a second person. if it is still too large, place a long spade bar between the concrete and the wall of the hole. Pull down on the handle and roll the concrete up and out of the hole.
Place the concrete onto a cart with wheels and take it to the location where you will discard it or recycle it.
If the lump, slab or footer is too large for several people to move after digging around the concrete, use a rented jackhammer to break up the concrete into smaller, manageable pieces.