It's where a river's level rises very fast, the flood may arrive instantly in the form of a wave or wall of water. People have no time to get to the safety of higher ground.
Flash floods are unusual in the UK, we do get a lot of rain and sometimes it's really heavy but normally the soil and plants act like a giant sponge. They soak up the rainwater and slow it down on its journey into our rivers.
Sometimes the sponge effect doesn't work because the ground is already very wet. When that happens all the rain tries to get into the river at the same time. If the river isn't big enough to handle the water there will be a flood.
In a flat area the rainwater sits around in temporary ponds and lakes until there is room for it to get into the river, it's like a rainfall traffic jam.
Where the ground is steep you could be looking at a dangerous flash flood. Gravity pushes all the rainwater into the river at the same time. The river level rises really quickly, and the water moves downhill fast. As it travels it picks up objects in its path. They might be cars, trees or even giant boulders.
Steep narrow valleys are the most at risk, a lot of water is trying to push through a small gap. This causes a 'bottleneck' and makes the flooded river even deeper.