Suppose Jack wants to calculate the average time a person walks per day in his city.

It means to conduct the study Jack will have to ask each person in his city how much time they walk per day. Of course, Jack cannot ask every person in the city so what he will do he will ask few randomly selected people from his city, and after analyzing those values he can make an inference about all the people in his city.

a) Population: All the values that are the part of the study i.e. all the people in his city are the population for Jack's study.

b) Sample: Some values selected from the population on random basis i.e. few people that Jack has selected as he cannot ask every person in his city.

Few values collected from the population are not the complete population but the part or the sub-set of the population. It can also be said that these few values represent the population approximately thus can also be known as the sample.