The Uncertain Areas are what might be loosely described as "places" which exist only under high improbability conditions and were created on the starship Heart of Gold due to the Infinite Improbability Drive. During Arthur Dent and Ford Prefect's first trip on board the Heart of Gold they witnessed many strange things happening, due to the Infinite Improbability Drive being in use, including wild horses thundering through the sky which were taking fresh supplies of reinforced railings to the Uncertain Areas. Arthur and Ford witnessed this while residing on a strange version of Southend-on-Sea.
The Uncertain Areas were only mentioned this once and not much is known about them beside the fact that they can appear during highly improbable moments. They may be similar to the Third Reach of the Unknown, which is another uncertain and improbable area which can also appear while the Infinite Improbability Drive is in use, as observed by Ford.