Fiestón is an augmented form of the common Spanish word fiesta (party), meaning a "big party" or an "epic party." The suffix -ón is used to emphasize size or intensity.
In the context of "Se Me Olvidó," the word fiestón perfectly captures the song's theme of forgetting heartbreak through a lively and extravagant celebration. It's a fun, colloquial word that immediately conveys the energetic and carefree atmosphere the singer creates to move on from a breakup.
Christian Nodal, a Mexican artist, brings us a fun song about getting over a breakup, even when it starts with a lot of pain. Our singer is feeling pretty down, with a "soul hurting bad" [alma bien dolida] after a breakup. So, what's the cure? Beers with friends, of course! He calls up his buddies, they get a truck, pick up some girls, and suddenly, it's a party!
And guess what? It works! He realizes he didn't love his ex as much as he thought because, in the middle of the party, a pretty girl kisses him, and he forgets all about being heartbroken. He's dancing, not missing her anymore, and even decides that being single is starting to grow on him. He's not going to beg her to come back; he's too comfortable for that! He even says, "And that's how I forgot you, my queen!" [¡Y así mero te olvidé, mi reina!]. It's a pretty blunt, but also kind of funny, take on moving on.