Fresa literally means "strawberry" in Spanish. However, in Mexican slang, it's also used to describe someone who is preppy, snobby, or privileged, often associated with a certain social class.
In this song, the artist uses "fresa" to describe a person who might seem a bit superficial or only interested in certain things, playing on the double meaning of the word. It's an interesting word choice because it highlights a cultural nuance that goes beyond its literal translation.
This song is a super fun, flirty track where the singer is completely smitten! He admits he was born into a tough life ("él nació duras" [he was born hard]), but now he's totally in love with every part of this person ("a cada parte de mí está en amores" [every part of me is in love]). He's so into them that he feels like he's always on their mind, even if they don't realize it yet: "todo el tiempo estoy en tu cabeza" [all the time I'm in your head].
He jokes about not having a lot of money ("plata no es lo que llevo entre las manos" [money is not what I carry in my hands]), and even makes a funny, slightly cheesy line about being a carnivore but their "corazón mexicano" [Mexican heart] winning him over, making him want to eat only plants and grains! He's so confident they'll end up together, he's already picturing them as old folks, arguing but still together: "vamos a estar juntitos de ancianos" [we're going to be together as old people]. He's basically saying, "I know I'm not right for you, but I'm so devoted, and no one else kisses you like I do, so I'm always on your mind!" It's a playful declaration of love and obsession.