- **Example Calculation (Sum)**: For a word count, if the key is `"word"`, the reducer would receive all the `1`s associated with that word and sum them up, giving the total count for that word.
- **Other Calculations**: The reducer might perform other aggregations like finding the maximum or minimum value, averaging, or concatenating values depending on the task.
In summary, the reducer aggregates or processes each key’s values returned by the mapper function, completing the overall transformation.