To access a specified element of a list by position use
Indexis the position of the element you want to access starting at 0, so the first element has index 0, the second index 1 and so on.
["First", "Second", "Third"]
This creates a simple list of three elements and returns the first one.
p is Person
This returns the first nationality a person has.
If the person has no nationalities this rule will fail. So if you don't know if a person will have nationalities at all add checks. E.g. check if the list is long enough:
if count(p.nationalities) > 0 then p.nationalities else 'No nationality' end