a cordon of policemen是什么意思？

a cordon of policemen

警察组成的警戒线

cordon

vt. 封锁，用警戒线围住；n. 警戒线，绶带，饰带；

例句
Police formed a cordon between the two crowds.
警察在两群人之间筑起了一道封锁线。

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。

policemen

n. 警察，（男）警察( policeman的名词复数 )，捕役；

例句
Eight policemen then spent two years combing through the evidence.
之后8位警察花了两年时间搜寻证据。