mmmm… is a creative collaboration between Emilio Alarcón, Alberto Alarcón, Ciro Márquez, and Eva Salmerón and they explain a bit more about it here:
BUS is made with wood and steel, materials that are typically used to build urban furniture. The three letters of BUS are big enough to accommodate two to four people each and protect them from rain, sun, wind, and inclement weather. They allow people to assume different postures of sitting or standing while waiting for the bus. The S allows people to lie back while they wait, and the B provides shelter.