Avioncito literally means "little airplane," as it's the diminutive form of avión (airplane).
However, in this song, it's used as slang associated with drinking tequila. The phrase "hacer el avioncito" refers to the ritual of taking a shot: salt, tequila, then lime. Snow Tha Product pairs tequilita y avioncito to paint a vivid picture of drinking to cope with a breakup, making this a clever and culturally specific word choice.
This song is a fantastic breakup anthem! Snow Tha Product is talking directly to an ex who keeps calling her after they were the one who ended things. The whole time, she's asking, "Que tanto me llamas / Si soy lo peor" [Why are you calling me so much / If I'm the worst?]. It's so sarcastic and powerful. She's reminding them of all the reasons they gave for the split: her lifestyle wasn't right, their family and friends didn't approve, and she was working too much to make them the priority.
Now that she's gone, she's choosing to move on in the best way possible. She's taking off with "Tequilita y avioncito" [A little tequila and a little plane] to see if life is better for both of them apart. She even throws in this amazing, half-funny, half-sad line about how she provided everything: "Compré casa, compré carro, compré perro / Y al final el pinche perro me dejó" [I bought a house, bought a car, bought a dog / And in the end the damn dog left me]. It's a classic song for anyone who's ever been told they're not good enough, only to have that same person come running back.