its the area code for the miami,florida area and lower.
Used by Rappers to refer to miami.. also called the MIA ( due to the fact that so many letters confuse them so they just use numbers)
also the most reconized area by the rap/hip-hop community
"305 till i die"
tourist ( to a ghetto miami person ):" where am i young man?"
ghetto miami person: " the 305 nigga "