Basically, I understand what the code is supposed to do but not sure what it means by a dictionary mapping a URL to a list of target URLs. Can someone perhaps give an example of this? 

```
WEB_DATA = os.path.join(os.path.dirname(__file__), 'school_web.txt')

def load_graph(fd):
    """Load graph from text file

    Parameters:
    fd -- a file like object that contains lines of URL pairs

    Returns:
    A dict mapping a URL (str) to a list of target URLs (str).
    """

    # Iterate through the file line by line
    for line in fd:
        # And split each line into two URLs
        node, target = line.split()
        raise RuntimeError("This function is not implemented yet.")

```