There is no proven scientific reason why conkers keep spiders away, but some sources speculate that the fruits contain a chemical that acts as a natural deterrent. This chemical, saponin, is most potent when the conkers are fresh.
Conkers are the seeds of the horse chestnut tree. Using them to keep spiders away by placing them on window ledges or in doors is an old wives' tale and there is no definitive evidence that it works. However, people have used this technique for centuries, according to the Guardian. Some companies even sell spider repellent spray containing conker extract to keep these pests at bay.