to gradually get nearer to someone, usually in order to attack them

The advancing soldiers closed in on the town. 前进的士兵一步步包围了镇子。

The hunt chased the fox until it was too tired and weak to run and then closed in for the kill. 猎狐队一直把狐狸追得精疲力竭跑不动了，然后再包围上去捕杀。