What does 就 是mean and how do we use it I have seen many times people using like in this sentence 就是关于爱情的 so what does 就是really means
就 has many meanings, In different context, '就是' could mean 'is exactly' ; 'then it is' ; 'just/ only'
他最怕的话题[就是]关于爱情的话题 = The topic he afraid the most, [is exactly] the topic about love
讨论完关于工作的话题, 接下来[就是]关于爱情的话题 = Finished discussing the topic about job, next [then it is] the topic about love
他什么书都看，[就是]关于爱情的他不看 = He read any kind of book, [just/ only] the ones about love he doesn't read
就是 denote some situation or things is true, you can think the component this way 就是[other description], where [other description] is description which is true. In your example, 就是关于爱情的(就是[other description], where [other description] is 关于爱情的) ((something) is about (other thing) of love). 关于爱情的 is true. It is used to show that something is about other thing of love.
就是 could of course be used on its own in everyday conversations as an exclamatory comment, like, when someone says, 呆在家里以防止冠状病毒covid-19传播是非常无聊的, and your comment, 就是！！！
呆在家里 = staying home
以防止 = to prevent
冠状病毒 = (Guānzhuàng bìngdú) = Coronavirus
Covid-19传播 = Covid-19 spread
是非常无聊的 = (shì fēi cháng wú liáo de) is very boring.
就是！！！ = exactly!!!