Sharing your idea for the film and the story of the film is called "pitching" and if you want to be a filmmaker you need to get good at it.
Describe your film in a sentence. Few have done better than the Alien description of "Jaws in space." But beware - "movie x in setting y" or "movie a meets movie b" descriptions (known as "High Concept") are becoming cliched. Don't try and force your one-line description into one of these if it doesn't fit.
Be as precise as possible with your language. When pitching a film you need to use visual language. Make the audience of your pitch "see" the film you are describing - sell the sizzle not the steak.
Practice your pitch. Don't memorize it, but learn it well enough to pitch your family, friends and coworkers. Listen carefully to their feedback, particularly if they tell you they're confused about any part of your pitch. If the producer you pitch to gets confused, you will not sell your screenplay.