The West's calendar shows the history and time period of "The West." To the Japanese and Chinese, this era is known as the Christian Era. It also used to be used as a way to refer to the Western Calendar, though we usually just say カレンダー nowadays.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
What year in the Western calendar did the Showa era end?