A dark spot on your toenail may be something as simple as a bruise due to trauma, or a mole or other benign pigmentation on the nail bed, or it may be the start of a fungus.  In rare cases, it may be the sign of something more serious such as a cancerous lesion.  If you have any questions regarding the dark spot, your best bet is to have it examined by a doctor and don't leave it to chance.