The U149 in the title refers to the aspiring idols at the center of it all: who stand at under 149 centimeters tall. Together with the help of their similarly squat rookie producer, these idols aim to make it to the top! As the announcement card puts it:
"This story is about growth of little idols and their little producer."

(Source: Crunchyroll, edited)

Note: Announced during DAY 2 of "THE IDOLM@STER CINDERELLA GIRLS 10th ANNIVERSARY M@GICAL WONDERLAND!!!" final tour concert, a TV anime adaptation of the spin-off manga THE iDOLM@STER Cinderella Girls: U149.