Build Plate Temperature is the temperature of the heated bed during printing.
Heating the build platform keeps the material slightly fluid and viscous. Some materials form crystals as they cool and therefore shrink significantly during the cooling process. Heating the build platform keeps the material just above its solidification temperature to prevent this shrinkage. Heating the build platform also keeps the liquid plastic viscous, which improves the adhesion of the print to the build platform.
However, if the build plate is kept too hot, the print will be very fluid where it touches the build plate. This causes the material to sag a bit, causing "elephant foot" on the bottom side of the print. Heating the build plate will also create a temperature difference between the material which is resting on the build plate, and the material higher up in the model, which causes warping when the higher up material starts to shrink.