A more thorough explanation:

A joint inventor is a person who works together with one or more people to create an invention. When applying for a patent, all joint inventors must be identified on the application.

For example, if two friends work together to create a new type of phone case, they would both be considered joint inventors and would need to be listed on the patent application.

It's important to note that just because someone helps with an idea or does work for the inventor, it doesn't necessarily make them a joint inventor. However, there are situations where someone could become a joint inventor or even the sole inventor, so it's best to consult with a patent lawyer to be sure.