"Low downforce" means that there is not much air pressure pushing down on a vehicle, which allows it to move faster but makes it harder to control. Full definition
As if an extremely rough track wasn't enough of a challenge, an added wrinkle presents itself in the form of a new low downforce aerodynamic package featuring a considerably shortened rear spoiler and rules prohibiting how much teams can adjust the rear - end housing to aid how easily a car can turn. (sbnation.com)
Toyota ran a third car in low downforce specification in preparation for Le Mans next month, but it was the two regular Toyotas that ended up duking it out for victory after the pole - sitting No. 1 Porsche faded during the race. (wtf1.com)
From here you have a scale to choose from such as a high downforce car for wet weather and high grip in corners, to low downforce for high speed circuits such as Monza. (ps3attitude.com)