The function replaces the second occurrence of the letter 's' with the letter 'i' in the input string.

[CODE]:
```python
def f(x):
    count = 0
    result = ""
    for char in x:
        if char == 's':
            count += 1
            if count == 2:
                result += 'i'
            else:
                result += char
        else:
            result += char
    return result
```

This code iterates through each character in the input string and keeps track of the number of occurrences of the letter 's'. When it encounters the second 's', it replaces it with 'i' in the result string.