A bimodal distribution is a chart of frequency that has two different peaks or modes. The term mode here refers to a local high point of the chart and is not related to the other common usage of "mode," which refers to the most frequent number found in a distribution.
An example of a bimodal distribution would be a population with a large number of teenagers and a large number of elderly but a small number of middle-aged people. A frequency graph of this population based on age would show a peak near the beginning and another near the end.
When the two modes are different heights, the larger one is called the major mode and the smaller one is called the minor mode.