The figure above shows a graph that is known as the “bell-shaped” or Gaussian curve. It represents a “normal” distribution of a quantitatively variable character, which might concern height in men, or the level of resistance in plants. This variable is represented on the horizontal axis of the graph, and it ranges from the minimum to the maximum. The percentage of the population, which possesses a particular value of that variable, is represented on the vertical axis of the graph, with the highest percentage at the top. The dotted line is the mode. In a symmetrical curve, the mode is also the mean, or the average, and it is exactly halfway between the minimum and the maximum.
You will encounter very often with normal distribution in biology problems, so try to understand it.
Thanks to http://www.pearsonhighered.com/sullivan/sul_alg_trig/ and Return to Resistance: Breeding Crops to Reduce Pesticide Dependence By Raoul A. Robinson.