When an earthquake, volcano or landslide happens on the ocean floor, water is displaced. This water forms the start of the tsunami.
When the waves reach shallower water:
their height can increase by several metres
the shallow water slows the wave
the waves get closer together
It is hard to see that a tsunami is approaching. The most obvious sign is the coastal water retreats just before the waves reach the shore. This is actually the trough of the wave following behind.