A filing date is the date a patent application is received by the US Patent & Trademark Office (USPTO). Filing dates are important in patent law because they are often used to determine who invented something first and therefore able to hold patent rights to it. The famous story tells of Alexander Graham Bell rushing to the patent office to beat Thomas Edison in patenting the telephone in 1876.
Filing dates also establish when your patent rights begin. If the USPTO approves your patent application, you obtain patent rights starting from your filing date, not from when the application is approved.
A provisional patent application can quickly and inexpensively save a filing date before you decide to go ahead with a full non-provisional patent. If you file a provisional patent application on January 1, 2000, you have 12 months to file a non-provisional application. If you file the non-provisional on December 31, 2000, you could claim the priority date of your provisional application. If your patent application is approved and you are issued a patent, you would obtain patent rights for your invention from the provisional filing date to the non-provisional filing date and then 20 years from the non-provisional filing date.