Like others say, it's scale (hard water build-up). They are supposed to have a time to clean the scale light, but in my experience mine needed cleaning even when the light didn't come on. Vinegar usually works, but I did have to break down and clr then vinegar my old one.. One day it seemed like it was brewing a bit slow so I thought I'd bang on it trying to break the scale free. Bad idea, broke the electronics and it wouldn't turn on. I ordered a new Keurig and had a water softener installed. Not a single issue since.
In addition to running vinegar or clr/vinegar, you can also try cleaning the puncture needle with a paper clip or light air pressure through the over flow hose (google specifics for these two things).
Love my Keurig.