In Gen. 3:15 we see the first hint of someone to come and deal with the serpent, Satan.
God said it, and therefore it will take place. That is good for us. God never breaks His promise, never breaks His Word.
Through the ages God spoke to mankind through what is called Prophets. When God speaks to mankind we call that a prophecy. In ancient days, Prophets would bring a message from God to people whose immediate circumstances revealed a need for correction, hope, or an insight into the future.
Through Prophecies, God spoke about this one to come. God gave His Word on this and because His kept His promise, we can have great peace and confidence.