At one time, inventions could be protected only by filing a FULL patent application. Because this is a lengthy process, you may not want this delay at the beginning of your project.
You may still be developing your idea and not yet be ready to file the full application. But, you want to know that while you work on your invention, no one else can file a patent and get ahead of you.
The U.S. Patent Office now permits you to file a PROVISIONAL patent application. This is a simpler document that costs much less than a full application.