The function replaces each letter in the input string with the letter that comes two positions ahead in the alphabet. It does not modify any numbers in the input.

[CODE]:
```python
def description_approximation(input_string):
    output_string = ""
    for char in input_string:
        if char.isalpha():
            if char.islower():
                output_char = chr((ord(char) - ord('a') + 2) % 26 + ord('a'))
            else:
                output_char = chr((ord(char) - ord('A') + 2) % 26 + ord('A'))
        else:
            output_char = char
        output_string += output_char
    return output_string
```
You can test this approximation using the following code:

```python
inputs = ["hello", "world", "123"]
for input_str in inputs:
    print(f"Input: {input_str}, Output: {description_approximation(input_str)}")
```

This should give you the same output as the original function.

Let me know if you need any further assistance!