They also impede backmixing of sand.是什么意思？

They also impede backmixing of sand.

同时也可防止砂子的返混.

they

pron. 他/她/它们，人们，大家，政府，当局，她们；

例句
The storms had abated by the time they rounded Cape Horn.
他们绕过合恩角时，暴风雨已经减弱。

also

adv. 也，同样，并且；conj. 另外；

例句
The media have also abetted the feeling of unreality.
媒体也在一旁对这种非现实感推波助澜。

impede

vt. 阻碍，妨碍，阻止；

例句
At no time did I make an attempt to impede any investigatory effort.
我绝没有试图妨碍过任何调查工作。

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。

sand

n. 沙，沙滩，沙色；vt. 撒沙于，用砂纸磨光；

例句
... the mile of white sand beach bordered by palm trees and tropical flowers
四周种有棕榈树和热带花卉的长达1英里的白色沙滩