Courses are of two years' duration.是什么意思？

Courses are of two years' duration.

课程为期两年。

courses

n. 课程，路线，前进( course的名词复数 )，进程，课程，方针；

例句
It is probably easier to make an analogy between the courses of the planets, and two trains travelling in the same direction.
将行星的运行比作两列火车朝同一方向行驶可能更容易些。

are

vi. （用于第二人称单复数现在时，第一、三人称复数现在时）是；n. 公亩（等于100平方米）；

例句
We are scared to abandon ourselves to our feelings in case we seem weak or out of control.
我们不敢放纵自己的情感，以免显得太过软弱或缺乏自制。

of

prep. 关于，属于…的，由…制成；aux. 助动词 [非标准用语、方言] =have [主用于虚拟语气]；

例句
He did have a real knowledge of the country...
他对该国的确知之甚多。

two

n. 两个，两个东西，两点钟，一对；adj. 两个的，我

例句
He has been absent from his desk for two weeks...
他已经有两个星期没有坐在办公桌前了。

duration

n. 持续，持续的时间，期间，（时间的）持续，持久，连续，[语音学]音长，音延；

例句
Almost all workers heeded a call by the trade unions to stay at home for the duration of the strike.
工会号召工人在罢工期间呆在家里,几乎所有工人都响应了这一号召。