Hi Hiral Thaker - the toadstools are the fruiting body of underground fungi and have appeared because of the current weather conditions and time of year. As the weather turns cooler they will cease to grow.
If you wish to remove them to enable you to mow the lawn just brush them away with a stiff brush or scoop them up and add them to your compost heap where they will break down and add to the nutrients.
As Gary has said, they play an important part in the ecology of the garden, and they are beautiful. 