What does double bars (||) mean in SQL?

I’m trying to understand the following query:

select count(distinct Name || '' || Surname) from PEOPLE;

What does the double bars mean? What does this query do?

In MySQL:
select "aaaaa" || '' || "bbbbb";

+--------------------------+
| "aaaaa" || '' || "bbbbb" |
+--------------------------+
| 0 |
+--------------------------+


Answers:

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Method 1

double bars are concatination:

select 'hello' || ' ' || 'world' from dual;

yields

'hello world'

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