They are a continuous set of data.
It shows that the bar doesn't have any gaps between them. The formula to calculate them is Frequency Density= Frequency of Class Interval /Width of class Interval.
Grouped Data and Frequency Diagrams: In grouped data, data is shown into groups for example '25<x<or=to 35. This means that the value between 25 and 35 not including 25 but including 35. This data can be shown on a Frequency Polygon.
Whenever a question has the words Calculate an estimate of the mean you add to columns to your table or data.