Limousine and trucking companies are always their fronts.是什么意思？

Limousine and trucking companies are always their fronts.

高级轿车和运货卡车服务公司常常是他们的掩护机构.

limousine

n. 豪华轿车，大型豪华轿车，接送旅客的交通车；

例句
A limousine was sent to pick me up and deliver me to the set. Eat your heart out, Tom Selleck!...
他们派了一辆豪华轿车来接我去片场，你就眼红吧，汤姆·塞立克！

and

conj. 和，与，而且，于是，然后，因此；

例句
I spend a lot on expensive jewelry and clothing...
我花很多钱购买昂贵的首饰和衣服。

trucking

n. 货车运输，货车运输业；

例句
...the deregulation of the trucking industry.
解除对货运行业的管制

companies

n. 公司( company的名词复数 )，[军事]连，连队，客人；

例句
The workforce have recently been calling for their working hours to be reduced. Many companies have accordingly switched to a five-day week.
工人们最近一直在呼吁缩短工作时间。许多公司于是转成了一周五天工作制。

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。

always

adv. 总是，老是，永远，始终，常常，不断地；

例句
Some cities are always going to be more advantaged.
一些城市总会有一些得天独厚的优势。

their

pron. 他（她，它）们的，在提及性别不详的人时，用以代替 him 或 her；

例句
Their permissiveness toward their children reflects the wild abandon of their own lives.
他们对孩子的放任折射出他们自身生活的放浪形骸。

fronts

n. 前面( front的名词复数 )，前线，一面，幌子；

例句
People were stringing up decorations on the fronts of their homes.
人们正在各家门口挂起装饰物。