What does this colon (:) mean?

Question

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.

It is constructor chaining so the constructor with the subsequent : this call will chain to the ctor that matches the signature.

So in this instance

public ListNode(object dataValue)

is calling

public ListNode(object dataValue, ListNode nextNode)

with null as the second param via : this(dataValue, null)

it's also worth noting that the ctor called via the colon executes before the ctor that was called to initialize the object.

It means before running the body, run the constructor with object and ListNode parameters.

It calls the other ListNode constructor. You can do a similar thing with the base keyword to call a constructor of a class you're deriving from.