Usually, light rain will barely have an effect on the efficiency of flea killing products. However, in case there is a heavy downpour, there is a very high likelihood that the contents will be washed out, reducing the effectiveness of the treatment. Thus, you will have to apply the solution again.
It also depends on how long after the treatment the rain pours. If the product has already settled in the soil for a day, and it rains heavily, it will probably still be effective.
Besides, there are weather-proof solutions available on the market. It means that they can't be washed out by rain or affected by directed sunlight, so you won't have to worry about the weather at all.