Porosity refers to how well your hair is able to absorb and hold moisture. Here's some more info about porosity: http://www.naturallycurly.com/texture-typing/hair-porosity
You can do a couple of different tests to see what your porosity is. The Float Test: Take a couple of strands of hair from your comb or brush and drop them into a bowl of water. Let them sit for 2-4 minutes. If your hair floats, you have low porosity. If it sinks, you have high porosity. The Slip'n'Slide Test: Take a strand of hair and slide your fingers up the shaft (toward the scalp). If you feel little bumps along the way, this means that your cuticle is lifted and that you have high porosity. If your fingers slip smoothly, then you have low porosity hair.