A split means that there will be two versions of bitcoin. Its like a bamboo split into two. The first version is the BIP and the other is the Legendary. BIP is the new bitcoin while the Legendary is the old coin. After the split there is also a possibility that one version will be dissolved and only one version will remain. Thus the split can affect the bitcoin market greatly it either can make the market increase in value very fast or it will cause a tremendous price crash due to panic selling from the bitcoin holders. Thus before that time comes it is best to just convert your bitcoins to dollars so you can avoid any negative impacts that event will bring. And after everything is clear then you can buy bitcoins again without worries.