A tidal surge is an oceanic phenomenon with a couple of different applications.
The main meaning is when an especially high tide (usually due to a full moon) enters an estuary, and as the estuary narrows, the water rises higher and higher as it moves up the river, creating potential flooding problems for cities and towns built on the banks upstream. The Thames and the Severn in Britain are particularly prone to such flooding.
The other use occurs in relation to tropical depressions - or, as they are better known, hurricanes and typhoons. Such storms, due to their high wind speeds, create a very low atmospheric pressure, which literally "sucks" the water under them up higher than it would be. When such a storm strikes at high tide, the ocean water can be drawn several feet higher, causing much devastation.