Why is the movie called "8 mile"?

The film is named after a street in Detroit, Michigan that is officially Michigan Highway M-102, specifically a portion that follows the locally named "8 Mile Road".

In Detroit, particularly in previous decades, 8-Mile Road was a literal dividing line between two very racially distinct cultures. It runs along the boundary between two counties, one of which was very predominantly (~75%) white and upper/middle class, and the other was very predominantly (~75%) black and low/poverty class.

The street has thus become culturally significant, especially among the rap/hip-hop community. In particular, Eminem refers to the road a lot in his music, since he was spent much of his childhood in Detroit, and thus would be very familiar with the road.

8-Mile road is a major road in Southeast Michigan. In the Detroit area it has cultural significance because it is a defacto dividing line between the predominantly poor city area and the wealthier northern suburbs of the city.

https://en.wikipedia.org/wiki/M-102_(Michigan_highway)

I would call into question some of what @KutuluMike stated in his answer, and provide a better one - one from an authentic Detroiter.

Perhaps long ago it once served as a line that divided the Detroit culture, but I believe the inner-city culture has shifted significantly in recent years, for better or for worse. I grew up in the Metro Detroit area my whole life and not once did I hear it referred to as any line of demarcation, etc. In fact, I wouldn't even say there is a line necessarily. Just keep on driving north and eventually a black person is hard to find.

Also, I wouldn't say that 8-mile is that significant in the hip-hop community at large - only in Eminem's music. And for the most part, race is not a consistent theme to Eminem's music. What is a consistent theme is the struggles he has faced in his life: growing up in a severely impoverished area of Detroit and navigating the issues of raising children after all he has been through with his family, wife, etc. Sure, he is a white man in an industry nearly dominated by African Americans. But, if you actually listen to a lot of his music, it's not something he hits on that often.

This brings me to a point at which I will answer this question. The movie is called 8-mile because it is where Eminem grew up - the section of the road that the movie is referring to (it's a big road and there's actually some nice parts of it) is stricken with poverty and total societal breakdown. From the perspective of someone who visited this road firsthand too many times: It is one of the dirtiest places in Metro Detroit, known explicitly for its trailer parks and white trash. So, it is pretty significant that someone like Marshall Mathers became one of the biggest lyrical geniuses of all-time from such an armpit of society.

The movie 8-Mile did get its name from the road in Southeast Michigan; this road does separate two counties and two cultures; the movie explores the interactions between these cultures.

The road got its name because it is 8 miles North of downtown Detroit. Some roads have been renamed; that said, 8 Mile Road is one mile North of 7 Mile Road and one mile South 9 Mile Road. What might be called 0-Mile Road or the North-South divider is actually Michigan Avenue in downtown Detroit or Ford Road as one travels further West.