Enormous amounts of water
The sea is influenced by gravity - the play between the earth and the moon - which means it rises and falls twice a day. The differences in tides can be very large at the Wadden Sea - as much as 180 cm. That's in height - not in length! In stormy weather, the difference can be as much as 5 metres. This is one of the reasons why you see dikes in the Wadden Sea area.
The tides flood the tidal flats, of course - but also the roads to Mandø and sometimes the large west-facing beaches on Skallingen, Fanø and Rømø. Especially the big winter storms can cause the tide to rise all the way up to the dunes - in some places several kilometers from the sea.
The large amounts of water add both new material that is deposited and forms mudflats, and it can move around on the seabed. This means that where it was safe to walk the last time you were on the waves may now be quite unsafe - or completely changed.
So what do you do?
Understanding the tides: To keep track of the tides, you need to check the tide tables - you can find them on DMI.dk. They show both a time and a water level height. High and low tides change time every day - so always check before you head out. Next, you should also check the water level forecast - as the wind also affects the tide further. In particular, strong winds from the west/north-west over several days can affect the water level.
The best way to experience the tides is to go on a trip with one of the local guides - they know the wind, weather, water and tides.