More rays means more (accurate) reflections and stuff like that. With raydepth 1 you get only primary reflections. With raydepth 2 you also get secondary reflections (a reflection of a reflection) and so on. you usually don't need more than 6, unless you want your glass-textures to look like they're lamps without actually illuminating the scene without Additive turned on or when you have a huge render and you need the details like reflections of reflections of reflections of reflections of reflections of reflections of reflections of reflections of reflections of reflections. :-) When I'm rendering an animation I usually use raydepth 1 or 2.

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