In statistics, one of the most fundamental concepts is the population and sample. A population is all the member from a group. For example, if my population is the United States, I would have to collect data from everyone in the country. This is to say the least, very challenging.
To deal with this, must studies take a sample from the population. A sample is a portion of the population. Continuing our example, instead of collecting data from every in the US I would collect data from several hundred or thousand depending on the research question of my study.
There are several different techniques to sampling that will be covered later. For now, the most important thing to remember is that your research questions and circumstances of the study influence what steps you take. There is rarely one way to do this.