In a relative frequency histogram of ages of students, the height of each bar represents what quantity?

The number of students in the sample whose heights fall within the range defined by the cell or bin of the histogram.

Example:
Let the heights of students be represented by the sample
x = [55,62,58,70,63,57,65,59,61,56]
The maximum value is 70 and the minimum value is 55.
The range of the data is 15.
If 5 cells or bins are used for the histogram, each bin will have a range of 15/5=3.

The histogram of this data sample is shown below.
The first bin has a  range of [55, 58].  Its relative frequency is 3, which means that 3 students have heights within the range [55,58] inches.