What is the purpose of the keyword “||” in PostgreSQL? I'm new to PostgreSQL and I'm trying to figure out what does the operator || mean?

Google didn't help with that...
String concatenation as you can see here: http://www.postgresql.org/docs/9.4/static/functions-string.html

Array concatenation: http://www.postgresql.org/docs/9.4/static/functions-array.html

And others... It means: concatenation.