The function `f(x)` takes a string input `x` and appends the string "GOBo" to it if the input string is exactly "apple". Otherwise, it returns the input string as it is.

[CODE]:
```python
def f(x):
    if x == "apple":
        return x + "GOBo"
    else:
        return x
```

I apologize for any confusion caused. Please let me know if there's anything else I can assist you with.