Why Keplerian elements (a "Kep set" or TLE, two-line element set) are the six-plus orbital parameters that mathematically describe where a satellite is and where it is going: inclination, right ascension of the ascending node, eccentricity, argument of perigee, mean anomaly, and mean motion, plus an epoch time stamp. Tracking software takes these numbers and propagates the orbit forward to predict pass times, elevation, and Doppler shift. They are named for Johannes Kepler, who worked out the laws of orbital motion, and updated sets are published regularly because orbits decay and drift.
Watch out The Yagi answer plays on the word "elements" meaning antenna rods, but antenna elements have nothing to do with orbits; the magnetron and spread spectrum choices are unrelated filler.
Kepler = planets and orbits. If the question says Keplerian, the answer is about an orbit, not an antenna.