It means you should check out your environment a bit first. Typically used for people who come to a new workplace and wants to make changes right away. The legs do not reach down to the ground. He doesn't have a leg to stand on. Means the person has too lofty ideas, or consider him or herself at bit above other people.
Theory is one thing, reality another.
To get the showel under a person . A rigid person to deal with, possibly also a bit mentally slow. One havests as one has sown . A blubber burner . A stinky pipe.