To foretell is to predict or hint at something. Some people believe that two crows foretell good luck for the person who sees them.
Signs and omens foretell good or bad fortune, and symbolism in the early chapters of a novel can foretell events that happen near the end of the book. Longtime sailors sometimes say that a pink morning sky foretells bad weather later in the day. In each case, something is being foreshadowed or forecast. Fore is an Old English prefix that means "before" or "earlier," and it's combined here with tell, "to reckon or consider."