1. 控制;约束
To rein in something means to control it.
2. 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.
1. stop or check by or as if by a pull at the reins
e.g. He reined in his horses in front of the post office
Synonym: rein
2. stop or slow up one's horse or oneself by or as if by pulling the reins
e.g. They reined in in front of the post office
Synonym: rein
3. control and direct with or as if by reins
e.g. rein a horse