The Maro ability is a slang term used by Magic R&D to describe the ability that a creature's power and toughness are equal to the number of cards in its controllers hand. This ability is nicknamed after the first creature to have it: Maro from Mirage. In turn, this card was named after Mark Rosewater, the head designer of Magic R&D.
The ability is primary in both blue and green. Blue is the color of knowledge and green is the color of wisdom, so those are the two colors that like to care about what you "know".
The Saviors of Kamigawa expansion featured a cycle of "Maros": Kiyomaro, First to Stand, Soramaro, First to Dream, Kagemaro, First to Suffer, Adamaro, First to Desire, and Masumaro, First to Live.