A more thorough explanation:

The first-to-invent system is a practice in patent law where the first person to create an invention is awarded the patent, rather than the first person to file a patent application.

For example, if two people independently come up with the same invention, the person who can prove they came up with the idea first will be awarded the patent.

In the United States, an interference hearing is held to decide who is entitled to the patent if there is a dispute over who was the first to invent. This system is different from the first-to-file system used in other countries, where the first person to file a patent application is awarded the patent.