The mode is the value that occurs with the greatest frequency in a sample. NIST notes that it is not necessarily unique, and OpenStax calls a data set with two modes bimodal. It is the only common measure of center that also works for non-numeric data, such as the most frequent answer in a survey, but it can be misleading when the most frequent value is not representative of the rest.
How to find it
Count how often each value appears and pick the one with the highest count. If two values tie, both are modes.
Example
In 3, 4, 5, 6, 8, 8, 9 the value 8 appears twice and each other value once, so the mode is 8. In 430, 430, 480, 480, 495 both 430 and 480 appear twice, so there are two modes.
Compare it with the arithmetic mean (about 6.14 for the first list) and the median (6). The mean, median and mode calculator returns all three, and the guide to calculating an average explains when each is best.