It's when U define a certain Time/ Musical Grid, then u grab yer notes and u make they're start shift (move) 2 the closest grid line 2 each note. It's basically a way 2 correct the timmeng of yer performance 2 absolute mechanical perfection.
Then Steinberg Introduced this concept called Groove quantise, which basically means u can program a certain delay/ advance in some grid lines so that the notes become a bit more loose and funky... aquire a certain groove!!

Hope that clears it m8,

Z