It’s Spanish for “neighborhood.” In some countries, it’s used to formally define some geographic or political subdivision.
Another definition is for areas of a city inhabited mainly by Spanish speakers. From Wiktionary:
An area or neighborhood in a U.S. city inhabited primarily by people speaking Spanish or of Hispanic origin.
Why I Looked It Up
I had the heard the term before, and knew the basic meaning. In particular, I remember it being used in the lyrics of several songs from In The Heights (which is set in the Spanish enclave of NYC called Washington Heights):
From the song 96,000:
with 96,000, I’d finally fix housing
give the barrio computers and wireless web browsing
From the same song:
I’ll be downtown
get a nice studio
get out of the barrio
Another song is entitled Carnaval del Barrio.
Then, when on a missions trip to the Dominican Republic, I saw a sign that said “Barrio San Jose,” like the monument signs denoting neighborhoods in the United States.