The white wires are fitted in a container (or anywhere water can accumulate) at the level you wish the alarm to sound. When the water level reaches the bare parts of the wire the alarm sounds.
The white wires are 10 Metres long but can be extended or substituted for any type of wire and a length of 100 Metres has been used in testing.

PP3 Battery needed, NOT SUPPLIED

Taking a standard 9V PP3 Alkaline battery as an example (565mAh),

The alarm consumes 2.5mA when it sounds so it would sound for about 10 Days continuous.

This alarm consumes no power when not triggered so the PP3 would last many Years, probably 10.