A provisional patent is an "informal" patent application which is much shorter and simpler than a formal patent application.
Specifically, once an inventor perfects an invention, the inventor has a 1 year grace period to file a patent application.
If one feels he is running out of time on the grace period of 1 year, and is in danger of losing his rights to apply for patent protection,
one can file a "provisional" or "informal" application, which is a "short" version of a formal patent application.
This gives the inventor an additional year of time to prepare and file the formal application.
The inventor then will still get the benefit of a priority filing date that dates back to the date the provisional application was filed.
If a competing inventor later claims he was first to invent the invention at issue,
one can claim priority back to the date the provisional patent application was filed, thus establishing his priority of rights in the invention.
(In the U.S., the first to invent has priority, not the first to file, as in many foreign countries.)