SUBJECTS are the sources of your data. Most research in language-related fields uses people as subjects. Their characteristics, development, opinions, attitudes, knowledge, performance, etc. are used to answer your research question.
In order to choose appropriate subjects you need to decide what your population of interest is.
In research, population has a specialized meaning. Theoretically, a population is the group from which your subjects are drawn. Therefore, it is also the group that your subjects represent. When discussing your research findings, you must be careful not to generalize your conclusions beyond this group. For this reason, it important to identify the key characteristics of your subjects, and the population they represent. Are they children, teenagers, adults? Are they men or women. What is their nationality? What native language do they speak?
If you already have a sample group with which you will conduct your research, you need to identify their key characteristics carefully in order to produce a corresponding but theoretical population to which you can generalize your findings.
On the other hand, if you start with a large population that is too large to work with, then you must select a smaller sample from it. It is extremely important that this sample be representative of the entire group. Ideally, this selection is done through scientific yet random processes.