Stereo-A and -B are a NASA satellites that are taking images of the sun. If you looked down on the Earth-Sun line from above, you would find them here:
Stereo-A is named A because it is on approximately the same orbit as Earth, except Ahead. Stereo-B is Behind. So imagine we see a new sunspot, as viewed from the Earth. The sun is rotating, so then Stereo-A will see it, and then (assuming it hasn't disappeared) Stereo-B will see it, and then it will be back in view from the Earth. With these three views, we basically have full coverage of the surface of the sun. This is useful, because if a new sunspot appears on the exact opposite side of the Earth, we still get a warning, because we can see it in Stereo-B. This gives us our ~3 day predictor. [Sidenote: These two satellites are slowly drifting further ahead and behind (while still staying on an orbit similar to Earth's), so we are probably looking at slightly more than 3 days at this point.]
Here you can find the latest images from both Stereo satellites.
The Stereo satellites are also measuring the solar wind. Next up we will talk about the ACE satellite, which is measuring the solar wind just upstream from the Earth.