The Ken Combo refers to a spike setup with Marth invented and popularized by pro Ken Hoang.
The combo itself is simply as follows: single jump f-air, midair jump d-air, used to end beastly Marth combos with the infamous spike. However, some players refer to the entire chain of n-airs and f-airs followed by the spike as a "Ken combo" due to how epic combos of that kind were when Ken's Marth was the single dominant force in SSBM competitive play.
According to Ken himself, the Ken Combo was initially developed in the original Super Smash Bros. using Captain Falcon's spike.