The function maps input strings to their corresponding number words. It converts strings representing numeric values to their textual representation.

[CODE]:
```python
def f(x):
    number_words = {
        '0': 'zero',
        '1': 'one',
        '2': 'two',
        '3': 'three',
        ...
    }
    if x in number_words:
        return number_words[x]
    else:
        return 'Not a number'
```

Note: The number_words dictionary may contain mappings for all possible numeric values, but I have only provided a few examples in the code snippet.