The Kanji "mu" stands for naught, nil, nothing, or non-, un-, dis-, while the Kanji "getsu" stands for moon or month. So, the more-or-less direct translation of "Mugetsu" would be "moonless". The word "Mugetsu" is also a Japanese season word (specifically autumn), meaning "moonless night". It's also used to describe the moon when it's "invisible", covered by clouds.

Previously ichigo cant even cast a single hadou, now it looks like obtaining the final getsuga form he can cast spells, namely Mugetsu.