What Does MC mean in Eminem's song "No Apologies?"

master of ceremonies

"A master of ceremonies, microphone controller or MC (emcee), sometimes called a compère or an MJ for "microphone jockey," is the host of an official public or private staged event or other performance. The MC usually presents performers, speaks to the audience, and generally keeps the event moving. An MC may also tell jokes or anecdotes, use rhyming presentation style and otherwise interact with and be included in and be some part of the performance. The MC sometimes also acts as the protocol officer during an official state function."

"In the late 1970s, the term MC (master of ceremonies), also known as a rapper, became associated with the role in hip hop music and culture. An MC uses rhyming verses, whether pre-written or freestyled, to introduce and praise the DJ he or she works with, to hype up the crowd, to pay homage to his own stature, or to comment on society. As hip hop progressed, the title MC has been thought to mean a number of acronyms such as Microphone Controller, Mic Checka, Music Commentator, and one who Moves the Crowd, notably through Rakim's lyrics on the matter ("Eric B. easy on the cut and no mistakes allowed/ 'Cuz to me, 'MC' means 'move the crowd'"). Some use this word interchangeably with the term rapper, while for others the term denotes a conception and demonstration of the role indicative of skill and of connection to the wider culture, while the latter term does not.
Uncertainty over the acronym's expansion may be considered evidence for the ubiquity of the acronym: the full master of ceremonies is very rarely used in the hip-hop scene. This confusion prompted the hip-hop group A Tribe Called Quest to include this statement in the liner notes to their 1993 album, Midnight Marauders:
The use of the term MC when referring to a rhymer originates from the dance halls of Jamaica. At each event, there would be an announcer or master of ceremonies who would introduce the different musical acts and would say a toast in style of a rhyme, directed at the audience and to the performers. He would also make announcements such as the schedule of other events or advertisements from local sponsors. The term MC continued to be used by the children of women who moved to New York to work as maids in the 1970s. These MCs eventually created a new style of music called hip-hop based on the rhyming they used to do in Jamaica and the breakbeats used in records. MC has also recently been accepted to refer to all who engineer music.
MC Hammer began using MC even before his famous career started to mean "Master of Ceremonies" (while serving in the Navy during the early 1980s), as well as an abbreviation for emcee. In his ministry career, he later announced it meant "Man of Christ".
Most recently, the term "Netcee" (internet emcee) has taken the title role of MC in the hip hop subculture of online freestyle battling. The use of the term by standard hip hop artists though is that of a form of disrespect, claiming such persons are inevitably stuck in this role."
Source and further information:
http://en.wikipedia.org/wiki/Master_of_Ceremonies