Full explanation
Lean startup is a method popularized by Eric Ries. The idea is to build, measure, and learn in short cycles instead of writing a long business plan that no real customer has ever validated.
You launch a small version of your product, measure how people use it, and adjust quickly. You only invest more once an idea is proven.
It works especially well for non-technical founders, because you do not need to build everything before learning what people actually want.
Concrete example
Instead of building a full app for 6 months, you publish a landing page with a wait list. You see how many people sign up before writing a single line of code.