what is a Rainy Day ?

"....A rainy day is a day when the rainfall at a point received is above 2.5 mm in 24 hours...."

rainfall at a point ! what it means ? how can we measure that ?

we at most can take a beaker and place under the sky and collect water for 24 hours . beaker has scale on it, so we could measure how much cc has been accumulated. that should be a measurement.

but look they are saying, if the rainfall at a point exceeds 2.5 milimeter then we will call it a rainy day. i dont understand how they are measuring rainfall at a point. this is very confusing measurement system.
do you have any clue ?