These are special machines on wheels for transporting containers around a wharf. They drive over a container stack. They pick up containers either from a wharf or from the top of a pile of containers. Containers are attached to the straddle carrier using twist locks on the corners.
Straddle carriers drive containers to where they need to go. Some of these machines can stack containers very high.
Straddle carriers are another very expensive piece of equipment on a wharf.