Basically, "Hotboxing" is when you smoke weed via Joint, Blunt, Bong etc. in an enclosed room or vehicle. There are no spaces for the air to come out, so the weed stays in the area supposedly making you even more ripped because of the second-hand smoke you're inhaling along with the actual puffs.
But in reality, it really does nothing. The part in the weed that makes you high is THC. Ones you take a hit, you inhale all of the THC into your body, and none is blown back into the air. All of the second hand smoke does not make you higher, it just fucks up your lungs worse than normal. The only reason people thinks it works is because of the placebo effect.
Jamal: Yo we hotboxed my dads Cadillac yesterday, you should have reached!
Thomas: Well actually, hotboxing doesn't work, it is only the placebo effect changing your mindstate. You cannot get high off the second-hand smoke that contains no THC.