You may cast it on a later turn for its foretell cost (the mana cost listed as part of the foretell keyword). Foretelling is optional, so you can always just cast the spell as normal if you're looking for a more immediate impact. Foretell essentially splits the cost of a card over two turns.

What counts as casting a spell MTG?

To cast a spell is to take a card from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Previously, the action of casting a spell, or casting a card as a spell, was referred to on cards as “playing” that spell or that card.

Is foretell an activated ability?

Foretell is a keyword ability. If a card in your hand has foretold, you can pay 2 mana of any color to exile it face down. You can cast in on a later turn for its forecast cost. You can also just cast it for its regular mana cost, if that suits you better.