You should think of the '|' as an operation delimiter. YOu have your base search :
index=main airport="aus"
This returns events in the index named main, with the key value pair airport that has the value "AUS". And nothing else.
Now if you wanted to perform an operation on these search results, you need to '|' them to another function. Such as top.
index=main | top limit=10 airport
That will look in the index main, and return the top 10 values for the field airport. E.g. AUS=100, US=79, CHINA=40, etc.
You should read through the Splunk documentation : http://docs.splunk.com/Documentation/Splunk/6.4.1/Search/Aboutsearchlanguagesyntax
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