It is what it says on the box. MovieStim3 objects are gigantic RAM hogs, especially if they have audio. If you’re loading one per trial, my recommendation would be to make sure to delete each one at the end of the trial and try to free up the memory it was using. Other than that, all you can really do is get more RAM or shrink the movie files by reducing their resolution or framerate.
See also: MemoryError or Too many open files with MovieStim3