It means: what do the divisions on the horizontal and vertical axes represent.
For example in a graph of distance against time, the measure of each division of time could represent one second, while the measure of each division of distance could represent one metre. When the graph is drawn (for example, a curve or sloping line) the slope (gradient) of the graph represents speed at any given time. The units of the speed would be in terms of the units of measurement on the axes, that is, metres per second (if the units were seconds and metres), miles per hour, kilometres per second, etc.
The units could also be multiples of a standard unit, for example, 1000s of dollars (in a money graph). The scale is chosen so that the graph is meaningful to anyone reading it. The scale is also chosen according to the size of paper or screen on which it's displayed.