Glow is a flashy tool for your special effects palette. Increasing this value creates a glowing colored aura around the selected prim; the color is determined by the prim's color. To alter a prim's glow setting while editing it, go to its Texture tab and set a Glow value between 0.0 and 1.0.
You'll also find that the sun and moon can cast dramatic, glowing shimmers on the water!! To see this, you must enable basic shaders in Graphics Preferences.