Solar panels perform at their best on a clear sunny day with no cloud cover. This does not mean that the conditions are going to be favorable all the time. Most often panel performance is reduced due to weather conditions like cloud cover or rain. In such conditions, solar panels produce their power from diffused solar radiation (radiation scattered by molecules in the atmosphere) as there is hardly any direct sunlight.
As a general rule, there is a considerable decrease in solar panel performance due to cloud cover or rain. If the cloud cover is very heavy, solar panels may produce only 10-25% of the energy they normally produce. But it may be noted that some solar panels are designed to work well with diffused solar radiation on a typical overcast day.
Rains can also be helpful in some ways. For example, rain cleans the solar panels by removing dust and dirt which otherwise affects solar panel performance. And this decreases maintenance which is normally a problem in the case of rooftops.