The diagrams compare two different methods of defence for homes which are at risk of being flooded.
Overall, the key difference between diagrams is that they show flood protection with and without stopbanks. In either cases, the houses are raised from the ground level.
The first diagram shows a stopbank acts as a barrier to stop river water from flooding the buildings. The stopbank is a small mound of sand next to the river that is higher than 100 year flood level, and prevents the river from bursting its banks. Nearby houses can be built on stilts to prevent flooding from rainwater, and a floodgate beneath the stopbank can be allowed to drain the ponding water.
Where there is no stopbank, there will be nothing to stop the river from flooding. In this case, the solution is to put buildings on stilts. The height of stilts is measured so that the floor of the house is 300 mm above the 100 year flood level. This measurement is called freeboard.