There are two factors. The first is that the Earth spins on its own axis every 24 hours and the second is that the moon rotates around the Earth once every 28 days. Now imagine that you are the Earth and that someone else is the Moon. If you start off facing the Moon and then you both start to move, you will get back to your starting point 24 hours later. However, the other person will have moved on - in fact they'll have already covered 1/28 of their monthly journey. It will take you about another 50 minutes to catch up (1/28 x 24 = 51.53 to be precise). So, high tide will be 51 minutes later each day.