Fresón comes from the word fresa, which literally means 'strawberry', but in Mexican slang, it describes someone who is preppy, wealthy, or a bit of a snob. By adding the 'ón' suffix, it exaggerates the meaning to describe someone who is exceptionally flashy or bougie.
In the song, Peso Pluma sings "Dicen que soy fresón" (They say I'm very flashy) to playfully boast about his luxurious lifestyle, designer clothes, and expensive cars.
Peso Pluma's "RARI" is a pure celebration of living the high life. The whole song feels like a snapshot from an endless party. He's not thinking about the past or the future, just enjoying the moment with expensive cars, women, and weed. He tells us he's rolling around "En la Cadillac o en el Ferrari, rari, rari" [In the Cadillac or in the Ferrari] and that his luck has turned, saying "La ruleta ha girado / La suerte de mi lado" [The roulette has spun / Luck is on my side].
This isn't a quiet kind of success. It's loud and flashy, from the big Cuban link chain from celebrity jeweler Eliantte to the constant parties. He's always got a girl with him because "Todo es mucho mejor" [Everything is much better] that way. The party never stops, either. As soon as one joint goes out, the instruction is "préndanse otro de mari, mari, mari" [light up another one of mari(juana)]. It's a simple, direct song about having money and a good time.