1.原意是拉紧(马的)缰绳;勒缰（使马停步）
If you rein in a horse, you stop it or cause it to go more slowly by pulling its reins.
The horsemen reined in and shouted at the men behind to turn back...
骑手们勒缰停住，对身后的人大嚷，让他们回去。
2.引申为控制;约束
To rein in something means to control it.
His administration's economic policy would focus on reining in inflation...
他的政府的经济政策将侧重于控制通货膨胀。
3.keep a tight rein on严加控制;严格管束
If you keep a tight rein on someone, you control them firmly.
Her parents had kept her on a tight rein with their narrow and inflexible views.
她的父母观念狭隘而僵化，对她管束很严。
The teacher kept a tight rein on his students.
那位教师对他的学生管得很严。
We will have to keep a tight rein on expenditure in the next few month.
我们得严格控制最近几个月的开支。
4.give free rein to放任;不管;纵容
If you give free rein to someone, you give them a lot of freedom to do what they want.
give/allow sb/sth free/full rein
give/allow free/full rein to sth
对…不加约束；放任自由；充分表达（感情）
The government continued to believe it should give free rein to the private sector in transport.
政府仍然认为应该给予交通领域的私营企业以经营自由。
The designer was given free rein.
设计者可以自由发挥。
The script allows full rein to her larger-than-life acting style.
剧本允许她充分展现她那夸张的表演风格。