Emissivity as "the relative power of a surface to emit heat by radiation." Emit means to "throw or give off." So Low-E glass obviously is a special glass that has a low rate of emission. In other words, if there is a heat source inside your house (or outside!), the glass bounces the heat from that object back away from the glass. So, in the winter months, if you have Low-E glass in your home, much of the heat given off by the furnace and all the objects which the furnace has heated, is bounced back into the room. In the summer, the same thing happens but in reverse. The sun heats things up the air, sidewalks, driveways, next door neighbors bricks, so on outside of your house. This heat radiates from those objects and tries to get into your house. Of course, it tries to take the path of least resistance, that being the glass. With Low-E glass much of this heat bounces off the glass and stays outside.