This is the 807th most frequent Chinese word.
乘
"乘" means "to ride," "to take advantage of," or "to multiply" (in mathematics).
In this sentence, "乘" is used as part of the verb "乘坐" to mean 'to take or ride (a mode of transportation)'.
他乘坐地铁去上班。
He takes the subway to work.
Here, "乘" is used as a mathematical operation indicating multiplication.
这个数字乘以二就是答案。
This number multiplied by two is the answer.
In this sentence, "乘" is used metaphorically in "乘机" meaning 'to seize the opportunity'.