Mum's glasses

Still just mucking around with different settings on my new camera. Can anyone explain ISO to me? This one I took with ISO 6400. I noticed that the higher the ISO setting, the brighter the image gets. But I don't think it's that simple. I'm sure I could just google it, and I probably will at some stage, but I'm feeling pretty lazy right now XD

On a somewhat related note, it was/still is disgustingly hot and humid today, which I suspect is a significant contributor to my current laziness. But God I love air-conditioning.