In rap, I heard "How low can you go?" some times. What does it mean? Is this phrase some fixed form? and is this used in everyday conversation as well?
The phrase "How low can you go" is not usually encountered in conversation; it's an allusion, a line in Chubby Checker's 1962 recording Limbo Rock. It refers to a challenge dance called the limbo which requires the dancers to pass under a progressively lower bar on their feet while leaning backward, so "How low can you go?" is the literal challenge.
In Ludacris' 2009 recording How Low that surface meaning of the phrase shows up at a couple of points in the video, where one or more of the girls are actually performing limbo moves; but in the context of the song it's wrapped in several layers of double-entendre, such as low meaning 'vulgar', get down (and dirty) meaning 'participate enthusiastically in pleasurable or erotic activity', go down (on) meaning 'perform oral sex (on)', and doubtless more that I've missed.
But this was hardly new. The phrase is also employed to carry the prom scene in the 1971 musical/1978 film Grease to its orgiastic choreographic climax—see this YouTube clip at about 2:30.