Dave Johnson for Macworld:

I get a lot of questions about ISO—many photographers don’t seem to understand exactly what it does. Your camera’s ISO control determines how sensitive the camera’s sensor is to light. On most cameras, ISO starts at 100 and goes up from there; the higher the number, the more sensitive the sensor will be.

When photographers talk to me about things like this I imagine it’s like when I talk to them about my amp settings. However, this is a good article to help you get your head around ISO and what it means for your photos.