A value proposition is a clear statement of the benefit a product gives a specific customer and why it's better than the alternatives.
It answers three questions: who it's for, what result they get, and why this product rather than another or doing nothing.
It's the source your headline, tagline and sales copy are written from.
Why it matters for a launch
If you can't say it in a sentence, launch visitors won't work it out for you. A sharp one makes every listing and post easier to write.
Read more: How to write a product tagline