What does it mean "positive" or "negative" polarity in VGA's HSynch and Vsynch?
The sync polarity defines what is the idle and active state of the sync signal.
In your question, positive means the sync signal will idle at logic low level, and is shortly active on high level, creating a positive sync pulse.
In comparison, the industry standard VGA 640 x 480 @ 59.94 Hz, both sync polarities are negative, so the sync signals idle at logic high level, and shortly go active at logic low level.