Active PFC is 'Active Power Factor Correction'. I won't go technical because, firstly it's extremely complicated, and secondly there's just too much to even attempt to take in!!!
Simply put, PFC makes your power supply more efficient, compared to a PSU without PFC.
PFC maintains power factor so that it is as close to 1.0 as possible. More information available here, in somewhat basic form: http://www.memonline.com/pfc1.html
If I were you, I'd go PFC...!