VFD? - what is this thing

greetings,

i have a bridgeport boss 5 with a 2 hp ,3 phase spindle motor on it.
i have been following a thread about a rotary phase converter and it
mentions a "VFD". what is a "VFD" and how is it wired? what other
items are needed to make one work? i have a rotary phase converter
but its kinda noisy and i could do without that. where can i buy one
and what do they cost? or shouldn't i bother?

thanks
jeff