Soliá is a cool, stylistic contraction of the word solita (all alone). It's a prime example of the informal, catchy slang you'll hear in modern reggaeton.
Solita is the diminutive of sola (alone), which softens the word. In the song, Bad Bunny uses soliá to paint a picture of a woman who has left a bad relationship and is confidently going out by herself, ready to have fun and be independent.
This song is all about a woman who is completely fed up with her relationship. She goes out by herself, "Llego soliá, salió sin las amigas" (She arrived alone, she went out without her friends), looking for an escape from all the lies. She's not sad, though. She's feeling herself, posting on her story and dancing all night because she's "casi soltera" (almost single) and has had enough.
Bad Bunny tells her to send a message to her boyfriend, and it's a blunt warning: "Que no te deje sola / Porque puede perderte" (Don't leave her alone / Because you could lose her). She's run out of second chances after too many disappointments. The song even throws in a great pop culture reference to make its point, saying "Que la Kardashian ya no está pa Kanye" (That Kardashian is no longer for Kanye), basically saying that if their relationship can end, anyone's can. She's a grown woman who won't be told what to do, and tonight, she's in control.