Mylar is a metalized plastic sheeting. It will reflect about 98% of the light that hits it. You should put it on the walls around your plants to reflect wasted light back to the plants. It comes in three different roll lengths and two different thickness', 1 mil and 2 mil. (a mil is 1/1000th of an inch) We recommend the 2 mil for the better workability and strength. It comes in 50'' wide rolls.