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".

Structure
就是 + adjective

Examples

tā jiùshì cōnɡmínɡ,kǎoshì zǒnɡshì 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.

wǒ zhēnde shénme dōu méi zuò,tā jiùshì duì wǒ yǒu piānjiàn。
我 真的 什么 都 没 做,他 就是 对 我 有 偏见。
I really didn't do any of it. He's just prejudiced towards me.

tā jiùshì piàoliɑnɡ,méishénme biéde yōudiǎn。
她 就是 漂亮,没什么 别的 优点。
She's just pretty; she doesn't have other talents.

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.

Structure

就是 + verb
Examples

wǒ lǎoɡōnɡ jiùshì ài chōuyān,tā ɡēnběn jiè bùliǎo!
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.

wǒ jiùshì bú qù,wǒ yíxiànɡ dōu bú ài qù jiǔbā。
我 就是 不 去,我 一向 都 不 爱 去 酒吧。
I'm just not going to go. I've never liked going to bars.

tā jiùshì bú xiānɡxìn xīnɡzuò,nǐ zěnme ɡēn tā jiěshì yě méi yònɡ。
他 就是 不 相信 星座,你 怎么 跟 他 解释 也 没 用。
He just doesn't believe in astrology. No matter how you try to explain it to him, it won't work.

To limit the scope of verbs and adjectives
就是 can be used before verbs and adjectives to limit the scope of what the speaker is talking about, eliminating options in order to focus in on something. This is a bit like "only", or "just" in English. This is quite similar to the first usage.
Structure
就是+focus
Examples

jǐnɡchá,nǐ kàn nǐ kàn,jiùshì tā,nǐ kuài diǎn qù zhuā!
警察,你 看 你 看,就是 他,你 快 点 去 抓!
(Emphasizing that the thief is him and no one else)Officer, do you see? It's him! Go and get him.

suóyǒu de rén dōu zhǔnshí dào le,jiùshì xiǎo yánɡ chídào le。
所有 的 人 都 准时 到 了,就是 小 杨 迟到 了。
(Emphasizing that only 小杨 was late, everyone else was on time)All of the other people arrived on time, it was just Little Yang who was late.

wǒ duì bié rén méiyǒu shénme yìnxiànɡ,jiùshì duì nǐ yìnxiànɡ shēnkè。
我 对 别 人 没有 什么 印象,就是 对 你 印象 深刻。
(Emphasizing that it was only you which made an impression on me)I wasn't impressed by the others, it was only you who made a deep impression on me.