Most American rap music is phrased in an English Dialect called African American Vernacular English (AAVE). This song is certainly in that category (which is why I added the AAVE tag to the question). Because it is traditionally spoken by poorer African-Americans, this hasn't traditionally been considered a "prestige" dialect. But it is a fully-fleged English dialect, in theory every bit as valid as Midland American or RP. These days it tends to get used as a cultural marker to show the speaker (of pretty much any racial background) is quite far from being a member of the USA's rich (and melanin-poor) elite classes.
There are lots of other names for this dialect floating around (eg: ebonics, Black English, Spoken Soul, or my favorite Jive), but they all tend to have a lot of racial or political baggage. If you want to talk about the dialect on its own terms, apart from any pre-judgement, best to use the proper linguistic term.
The word "rollin'" is short for the fairly common phrase "rolling down the street", which refers to the act of riding in a moving car (typically, but not always, as the driver). The dropping of the "g" is charactaristic of the AAVE dialect, although it is actually quite common in many American dialects (and affected dialects by politicians, not to mention posing rap MC's).
(Note: This started as a comment on coleopterist's answer, but got long. Other than his use of the word "ebonics", I have no issue with his answer)