VPD stands for Vapor pressure deficit, basically its the difference between how much total moisture the air can hold and the amount of water in the air currently, or humidity level. This is important because it controls how efficiently the stomata in the leaves exhale water, and in turn how all other processes are carried out. VPD that's out of whack slows transpiration and in turn everything else. They can't get rid of water, they can't move nutrients around and as a result they can't photosynthesize properly or grow like they should.