Tides are the rise and fall of sea levels caused by the combined effects of the rotation of the Earth and the gravitational forces exerted by the Moon and the Sun.

If you wish to read up on tides there is a good deal of information available here:
Wikipedia

Need a predicition for the next 7 days click here:
UK Admiralty Easy Tide

The National Tidal and Sea Level Facility provide tidal predictions for the next 28 days however the locations are limited, click here:
NTSLF