Okay, so let's say you have a group of friends who all like to speak a certain language called Tamil. They come from a place called Tamil Nadu in India, but some of them may have moved to other countries like Sri Lanka, Malaysia or Singapore where they can still speak Tamil with other people who like to speak Tamil.
So when we say Tamil population by nation, we are trying to count how many people in each country like to speak Tamil and identify themselves as Tamil. So we would go to each country and ask people if they speak Tamil and if they identify as Tamil.
Once we have counted how many people in each country like to speak Tamil and identify as Tamil, we can add all those numbers up to get a total Tamil population by nation. This helps us understand how many people in the world speak Tamil and where they are located.
So just like you have friends who like to speak Tamil, there are many people in different countries who like to speak Tamil too. And by counting them, we can know more about the Tamil population by nation.