Mean, sum, minimum and maximum of a list of numbers.
Add all the numbers and divide the sum by how many there are. For 4, 7 and 10 that is (4 + 7 + 10) / 3 = 7. The arithmetic mean is sensitive to individual very large or very small values, so a single outlier can drag it well away from what describes most of the data.
The median is the middle value once the numbers are sorted — half fall below it, half above. Unlike the mean it barely responds to individual extreme values. That is why salary statistics usually quote the median: a few very high incomes lift the mean but leave the median almost untouched.
The mode is the value that occurs most often in a set. In 2, 3, 3, 5, 9 the mode is 3. A set can have several modes, or none if every value is unique. The mode is the only one of the three averages that also works on non-numeric data — the most frequently chosen answer option, for instance.
If the data are roughly symmetric and free of outliers, the mean is the most informative. If the distribution is skewed or carries a few very large values — incomes, prices, waiting times — the median describes the typical case better. Use the mode when what matters is the most common value rather than the average one.