Un ratito is the diminutive form of un rato (a while), meaning "a little while". This small change adds a sense of intimacy, casualness, and fleetingness, which is central to the song's theme.
Bad Bunny uses this phrase to set the expectation for a brief, no-strings-attached encounter, singing "Esto es un ratito, Mami, no te acostumbres" (This is just for a little while, baby, don't get used to it). Understanding this key phrase unlocks the entire story of the song.
If you want brutal honesty set to a smooth reggaeton beat, Puerto Rican superstar Bad Bunny delivers it straight up in "Un Ratito". From the very first line, he refuses to sugarcoat anything: "Quizás yo vuelva con mi ex / Y no te vuelva a ver" ("Maybe I'll go back to my ex / And never see you again"). He is not looking for a fairytale romance, and he makes sure his partner knows it right away with the main hook, "Esto es un ratito / Mami, no te acostumbres" ("This is just for a little while / Baby, don't get used to it"). The word ratito comes from rato (a short period of time), using the diminutive -ito to stress just how temporary this fling really is.
Even though he admits his life is messy with "Mi vida es complicada... Un poco inestable" ("My life is complicated... A bit unstable"), he still enjoys every single second of their physical chemistry. He hypes up their connection with blunt lines like "Con cualquiera no me acuesto" ("I don't sleep with just anyone") and invites them to hit the dance floor to "perrear" (a classic Puerto Rican term for dirty dancing or grinding to reggaeton). In the end, he blames his own nature, concluding "yo nací pa' estar solo" ("I was born to be alone"). It is a fun, upfront track about enjoying the moment without pretending there is a future.