L – Lovable
Your users should enjoy using your product.
S – Simple
Your users should find your product straightforward and easy to use.
R – Refined
Your product should be released as soon as your users find it to be lovable and simple, then it should be iteratively improved.
How are these things measured?
Note how each of the above sections includes "your users". They are the ones who can tell you if your product meets these criteria.
Why these three words?
I highly recommend reading through those links to learn more.