What does this colon (:) mean?

It (along with the this keyword) is instructing the constructor to call another constructor within the same type before it, itself executes.

Therefore:

public ListNode(object dataValue)
    : this(dataValue, null)
{
}

effectively becomes:

public ListNode(object dataValue)
{
    data = dataValue;
    next = null;
}

Note that you can use base instead of this to instruct the constructor to call a constructor in the base class.