The adverb 就 "exactly"
就 has several meanings. Let's look at the meaning "just, exactly".
The adverb 就 
gives more emotional coloring to a sentence and confirms something that was stated before.
It is placed either before the verb 是 or before the verb 在 .
gives more emotional coloring to a sentence and confirms something that was stated before.
It is placed either before the verb 是 or before the verb 在 .Examples
这就是我丢掉的包！
This is the bag I lost!
zhè jiù shì wǒ diūdiào de bāo!
警察的工作就是执法
The job of a police officer is to enforce the law
jǐngchá de gōngzuò jiù shì zhífǎ
我的家就在学校后面
My house is just behind the school
wǒ de jiā jiù zài xuéxiào hòumiàn
就在那时，有人敲门
At that moment, someone knocked on the door
jiù zài nà shí, yǒurén qiāo mén