Question

how does prespiration or sweating keeps our body coolon a hot day

Answer 1

We all know that evaporation gives a coolinhg efect. In summer we sweat a lot
and hence when the sweat evaporates it gives a colling effect

Answer 2

It is based on the fact that evaporation causes cooling.This is similar to the cooling of water kept in a mud pot in summer.The mud pot contains very tiny holes through which the water seeps outside.It gets evaporated by the warm air outside.To get evaporated it requires some energy and this energy is the heat energy which is taken from the water itself.So wate is getting cooled.This is what happens in our body when the perspiration comes out through the skin holes. 
A wet bulb thermometer also works on the principle that evaporation causes cooling.