A solar eclipse occurs when the moon comes between the Sun and the Earth and thus casts a shadow on Earth. The shadow can be quite large – as you can see from the excellent image in the January 2, APOD.
There will be a partial solar eclipse tomorrow (January 4). Fermi orbits the Earth every 96 minutes: for two of those orbits tomorrow Fermi will pass through the shadow of the eclipse. This won’t cause any problems – each orbit, we pass through the nighttime and thus dark side of the Earth. Passing through the eclipse means that we will spend a little more time recharging the spacecrafts battery from the solar panels than we would ordinarily need.
The Fermi flight operations team closely monitors the performance of the observatory, they need to know if we will pass through an eclipse so that we won’t interpret the change in battery charging performance as a potential problem on the spacecraft.
It’s neat to think that a observatory designed to detect gamma-rays from the Universe can notice more classical local phenomena on Earth.