Now we are really getting into the meat of the matter. The ACE satellite is located just upstream from the Earth, so it basically measures the solar wind about an hour before it reaches us at the Earth. But why does the solar wind matter?
The aurora is essentially a result of a huge energy release. This huge amount of energy that gets pumped into Earth's geospace is coming from the sun, via the solar wind. So if we want to know when there will be a lot of geomagnetic activity, we need to know when the solar wind is going to be putting a lot of energy into the system.
This is the ACE plot I look at most often. You can follow links on that page to look at longer or shorter timescales and also at different variables. Here is how to interpret this plot:
In the top panel, the red trace shows Bz which is important. This is the vertical component of the sun's magnetic field (the sun has a magnetic field too!). The white trace shows the magnitude of the sun's magnetic field. A high magnetic field strength is good (say, over 6 nT). Generally, if Bz is high, but also flipping between north and south (large positive to large negative) this is a good thing.
The solar wind speed and density are also important. Ideally, for strong activity you would like to see a high solar wind speed and a high density. Unfortunately, if the speed is high, the density is usually lower, and vice versa. This is like cars on a road, if you let them go fast the spread out, but if they have to go slowly they get bunched up. For solar wind speed, 300 km/s is kind of a baseline. Getting up to 600 km/s is really good. For the density, 1 /cm^3 is baseline, and getting up to 10 /cm^3 is really good.
The NOAA page takes this information and tries to put it in an easy-to-understand "dial" plot, and they will have a little arrow pointing to the current value (like a car speedometer):
One of my favorite plots to look at combines the data from ACE and from Stereo-B. Check it out. This shows the solar wind density, velocity, and magnetic field from both satellites, only the data is shifted appropriately along the x-axis so that they should line up in time. This is a great way to see solar wind changes that might be observed at Earth.
Next up will be the GOES satellites, which give us around a one hour predictor.