What does it mean to say the Kepler's laws are empirical in nature?

'Empirical' means 'based in evidence and measurement'. describe the motion of planets in a solar system. By saying they are 'empirical', we are saying they are only descriptive, they do not have a theoretical explanation for the phenomena they describe.

Kepler's Laws were formulated before Newton's development of his theory of gravity. Kepler used mathematical rules to describe a large class of observations. He did not have a theoretical explanation for why the planets move as they do, but could describe their past motion and predict their future motion.

We sometimes say that theories in science must have descriptive, predictive and explanatory . Empirical laws like Kepler's are descriptive and predictive, but don't explain the phenomena.

It was only when Newton developed an understanding of gravitation that it was possible to explain the reason why the planets move as they do.