Imagine you and all your friends live in a big town. Each of you is a separate person, but when you all come together, you make up the population of the town. This population is called a population entity.
Now, if we talk about a singular population entity, it means that we are only considering one group of people as a whole, without looking at each individual separately. So, instead of looking at each person in the town, we are just thinking about the town as one big group.
It's like when you have a bucket of colorful toy blocks and you want to know how many blocks are in the bucket. You don't need to count each block one by one. Instead, you can simply say, "There is one bucket of blocks" without worrying about how many individual blocks there are.
In the same way, instead of looking at each person in the town, when we talk about a singular population entity, we focus on the whole group of people together.