This article is from the Digital Broadcast Satellites FAQ, by Brian Trosko email@example.com with numerous contributions by others.
The signal sent from the satellites to your dish is of a
frequency that is blocked by nearly anything, which is why
your dish must have clear line-of-sight to the satellites
in order to function. Water blocks the signal.
When it rains, this reduces the signal reaching the dish,
which reduces your overall signal strength. Severe storms
can knock out your picture all together, but this usually
doesn't happen, and if it does, it doesn't last for very long.