The steam is cooled in a heat exchanger. This makes the pressure lower on this side and ensures that the the steam keeps flowing. However, the heat exchanger only cools the steam down to 180°C (and 900 kPa). It can now be piped into the steam main as a useful resource.
The heat exchanger uses cold water from the river and this gets warmed up. However, instead of going back into the river, this preheated water can be fed into the steam generator - thus reducing the amount of waste.
A conventional power station produces steam by burning coal. Again, this steam drives a turbine and is cooled using river water or cooling towers. However, the warmed up cooling water is usually put back into the river, wasting the enrgy that was used to heat it up.