Story proposal probably means a lot of different things.
If they mean treatment, they're talking about a bare-bones version of a screenplay. No camera direction, no editing or soundtrack suggestions--just characters, dialogue, and character actions.
If they mean scriptment, they're talking about a halfway point between a script (which contains ALL information needed to make something into a movie/episode, including camera work) and the screenplay. A scriptment gives the writer a bit of input in terms of direction. Most directors will do their own thing either way.
Otherwise, "story proposal" could just mean a long pitch. From my understanding, most TV shows are conditionally signed on by networks after a proposal. The "front 13" (first 13 episodes) are signed on officially after a network views the Pilot episode and likes the product. The "back 9" (last 9 episodes) are signed on some part through the airing of the front 13 as the network tests the waters to see how the show's doing. The "story proposal" is just a pitch of all 22 hypothetical episodes, since it's doubtful the producers have screenplays actually written for the entire season by the time they've secured financial backing for a show (that'd be ~1100 screenplay pages!).
This is in a perfect world, mind you. Some shows air the front 13 and don't get signed on again until much later, at which point they just air a new season. This is why so few shows actually have a perfect 22 episodes in their first season.