a Tsunami is a massive wave or series of waves, sometimes 3 metres high, sometimes 30 metres high that devastates coastal regions and drowns people. The most common cause of a tsunami is an Earthquake and earthquake prone regions of the World are known. Tsunami can also happen due to massive landslides or asteroid impacts and those can happen anywhere where there is a large body of water.
English speaking people living, for example near the Pacific Ocean where tsunami happen may need these warnings. Even if you are an speaker living far inland or in a country like the UK where tsunami are unlikely you may need these warnings. Do you go on holiday anywhere where there is a danger of tsunami?
Sometimes the first thing that people see is an unusually large wave. A larger wave may follow the first one so don't stay and watch. Get yourself and your friends to high ground as fast as you can. Avoid streams and rivers as the wave can funnel up existing water courses.
Other times the first thing that happens is that the sea or ocean draws back exposing the dry sea bed. Curiosity draws people onto the exposed sea bed where they are killed when the waters return. The next stage in that type of tsunami is that the water returns with force and reaches higher than the normal coastline. If you are ever at a coast and you see the water retreat get yourself and your friends to high ground as fast as you can. The most common cause of a tsunami is an Earthquake. If you are ever at a coast and you feel an earthquake get yourself and your friends to high ground as fast as you can.
A roaring sound from the sea can also mean a tsunami is on the way so evacuate.
There can be danger of more waves for several hours.