Plebitas is a colloquial and somewhat regional term, particularly common in parts of Mexico, that refers to young women or girls. It's a diminutive of "plebe," which can mean common people or youth.
In the context of this song, the singer uses "puras plebitas bellas" (only beautiful young women) to describe the attractive women he enjoys being around. It adds a touch of local flavor and a casual, almost boastful tone to his description of his lifestyle.
The whole song is one man's running list of what he has and what he does with it, and he opens it with a bottle: "Destapa otra botella / Quiero brindar por ellas" (Uncork another bottle / I want to toast to them). The "them" is the plebitas, the girls, and he is not subtle about the arrangement. "Si de una me enamoro / Me la llevo a Paris" (If I fall in love with one of them / I take her to Paris), he sings, and then straight away: "Y si nomas me gusta / Me la llevo al hotel" (And if I only like her / I take her to the hotel). That two line swing is the joke the rest of the song is built on.
The chorus is where he reads out the inventory. A norteno band, this year's cars, "Pura Buchanita del sellito rojo" (Nothing but Buchanan's with the little red seal), the beaches, the clubs, the cockfight rings where he bets millions, and friends "de muy alto rango" (of very high rank) all over the place. He calls himself enamorado, a man in love, which is a generous word for what he is actually describing. Sergio Sanchez Ayon, who leads the band, wrote it, and he sings it with the straight face of someone who has not noticed the contradiction.