Of course the reason you've come to this location was to Seljalandsfoss, but most people don't realize that just a short walk from Seljalandsfoss is this waterfall hidden in a cave-like gorge. After visiting Seljalandsfoss simply follow the path north for a few hundred meters and you'll find Gljufrabui on your right side. There is only space for people to walk in one direction in/out, so make sure you're not walking in while others are trying to walk out. There's rocks you need to balance on in order to get in, and maybe hold the sides to make sure you don't fall into the water. Once you're in there's more space to move around.