就是 (jiùshì) can be used to add many different types of emphasis.
It can be used in the following ways:
To intensify adjectives
就是 can be used to modify an adjective to mean "really", or "really that and nothing else".
Tā jiù shì cōnɡ mínɡ ， kǎo shì zǒnɡ shì dì yī mínɡ.
他 就是 聪明，考试 总是 第 一 名。
He's just really smart. He's always gets the best grade on tests.
To express resoluteness
Used before a verb, this usage has the feeling that the speaker is being stubborn and is unwilling to change their mind.
Wǒ lǎo ɡōnɡ jiù shì ài chōu yān ， tā ɡēn běn jiè bù liǎo!
我 老公 就是 爱 抽烟，他 根本 戒 不了!
My boss loves to smoke, he simply doesn't listen to the warnings.