It refers to the first line of the lyrics of the Lucky Star opening (https://www.animelyrics.com/anime/luckystar/motteke.htm). Which refers in turn to a rule to have the hem of the skirt be 3 cm above the knees. Many girls, at least in anime, like to wear the shirt a bit shorter. In reality that seems to be less so (https://hinative.com/questions/5293323), but in the time of Lucky Star it was more popular than today (https://medium.com/@satokasotome/do-japanese-female-students-still-wear-short-skirts-in-winter-today-ac6db4bc4400).