orig.append(other):
orig and other have type List, the result is a List that consists of all values of orig followed by all values of other.

Look at the examples:

list1.toString() # "(3)"
list3.toString() # "(1 2 3)"
list1.append(list3).toString()  # "(3 1 2 3)"

Does that help?

I don't understand
The orig.append(other) method will create a list which is a concatenation of all nodes in orig and all nodes in other and which shares as many nodes as possible with orig and other (while still being a singly-linked list).

What is concatenation of all nodes?
All the values added together?

The solution doesn't work when being tested, but works when submitted that's not good