"Quantitative" data deals, literally, with quantities—that is, things that can be measured. Conversely, "qualitative" data deals with qualities—that is, things that can be observed and described, but not directly measured.
In sociology, quantitative data is often collected by amassing a large body of qualitative data. For instance, information about an individual's lifestyle and habits is qualitative data: it describes observations about a research subject. However, scientists collect lifestyle, habit, age, and demographic data from a large pool of subjects they can measure—for instance, how many people in a given demographic group act a certain way—and so this large data pool represents quantitative data.
Other sociological research methods are less easy to turn into quantitative data, however. An interview, for example, represents qualitative data that is difficult or impossible to compare in a large pool with other interviews in a way that would allow for it to be evaluated quantitatively.