The Bubble Floor is a gimmick in Mega Man V, found in Venus' stage. It is a cluster of large, bouncy bubbles that covers the ground.
Bubble Floors are included in the Megamix Engine, and it first appeared briefly in Make a Good Mega Man Level: Episode Zero, on the Floor 5 hub of Null and Void. It went on to make a similarly brief, yet much more involved, appearance in Make a Good 48 Hour Mega Man Level, in the Box Cartel stage Pandora's Parlor.
In MaGMML: Episode Zero and the current public build of the Megamix Engine, Bubble Floors will bounce Mega Man/Zero in the air if he holds the jump button as he lands on one, unlike how they worked in Mega Man V. The height of the bounce depends on the size of the bubble Mega Man's standing on, as well as how long he's been bouncing.
In MaG48HMML, Bubble Floors behave much closer to their Mega Man V appearance. Holding the jump button now has no bearing on how high Mega Man will bounce, nor can he bounce infinitely. Instead, after landing on a Bubble Floor, he will do a few short bounces on it before stopping, at which point he will be allowed to jump again. If he jumps from a large bubble, he will jump significantly higher, while jumping from a smaller bubble will more or less be the same as jumping normally.