It depends rather a lot on who I am talking to... which mostly means explain this as you would to a 7-year old, or "explain this as you would to my grandma."
In "general" my pitch goes something like "Steem is a social content environment similar to what you'd get if you combined Facebook, Instagram, twitter, YouTube, SoundCloud, Pinterest, WordPress and a few others under a single umbrella... but UNlike all of the above which have a big central corporation that makes money off your efforts, with Steem YOU have the opportunity to be rewarded for your efforts and even become part 'owner' in the system you are part of, a bit like a co-op."
I don't even touch on blockchains or cryptos and all that unless we get to the "Cool! Tell me more!" stage. If there's no follow-up, I just want them to take away a simple street-level explanation that might stick with them till the next time they see/hear the word "Steem."