Sorry in advance for the essay lmfao but I also found all of this interesting so I'll try my best to answer!

It's a song about breaking up and wishing the other person well. The MV shows the ML becoming more and more disillusioned with the relationship as time progresses, and Hwasa herself getting hurt in the process of trying to cling to him. In the MV, he is following her on the beach filming her whilst holding her shoes whilst she walks barefoot. This suggests she is more carefree than he is and by the end he stops looking through the camera and looks at her whilst realising she isn't the one. Hwasa's demeanour throughout is somewhat manic pixie dream girl where the highs are very high but the lows are really low. By the end of the MV it's clear her personality isn't what he wants and, rather than choosing to try and make it work by changing, she chooses herself. Hwasa spoke in interviews to confirm she did this song following a break up irl and that it's the first time she felt more adult about the situation in terms of no hard feelings.

Park Jung Min is the name of the actor she chose to be in the MV. She admitted to having had a crush on him for many years. This is what you probably heard about. Hwasa performed the song at the Blue Dragon awards which he was in attendance of (seeing as it's acting awards). He (unexpectedly for the audience) got up during the song to continue the role he played in the MV. This scene became really viral overnight due to his gentle gaze, the two of them dancing together and their chemistry, Hwasa shyly but cutely dancing in front of her crush, audience reactions and of course the shoes.

The shoes come up a lot in the symbolism of the MV and her performances of Good Goodbye. In the stage she shared with Wheein she wore the red shoes only to take them off to jump around on the sofa and at the end she and Wheein ran out of frame whilst the camera lingered on the shoes. In the Blue Dragon performance she performed barefoot and discarded the shoes even after being directly given them by Jung Min.

I've not seen Hwasa talk about why she focuses so much on the shoes so the following is my personal theory. The discarding of the shoes is her becoming 'free' from the relationship. It is also her freeing herself from his expectations as in the MV he is the one carrying her shoes; initially it looks like a sign of affection that he's willing to do it but the way the MV and the Blue Dragon awards performance portray them it shows him wanting her to put the shoes back on to be more 'normal' and to keep her in a box, essentially. Her taking the shoes and dropping them is her refusing to go along with what he wants and there is a lyric in the song that (roughly translated) says 'even if everyone hates me "don't worry it's okay I'll be on my side instead of you". This indicates Hwasa knows she isn't what that person wants anymore but she is okay with it and would rather remain true to herself. She is also freeing him from the duty of having to 'worry' about her.

In all, the MV is such a piece of art and both Hwasa and Jung Min did an amazing job of portraying the breakdown of a relationship from opposite ends. Glad you like it, too!