In the simplest terms it means "usefulness". Your invention must have "real-world use". What's interesting is that the invention must be useful at the time of the patent application. Even if some other technology comes
along in the meantime that makes your invention obsolete, your patent application is still valid under the usefulness test.
The USPTO says:
"Being new isn't enough nor is being different. For your invention to be patentable, it also must be useful. The invention must perform a function, do what you say it does, and benefit society in some way. A machine that doesn't perform its intended purpose is not useful, and therefore cannot be patented."