meaning of (array)function()

You are looking at type casting: http://php.net/manual/en/language.types.type-juggling.php

What the code does is caste the value returned by get_option() to an array. It is being done so that array_merge() works correctly and doesn’t trigger warnings/errors. That much is pure PHP and is off-topic. The only reason I chose to answer rather than to post a comment is because the return value of get_option() will return various types of data depending on context. You can see that noted in the source:

@param bool|mixed $pre_option Value to return instead of the option
value.

https://core.trac.wordpress.org/browser/tags/4.0/src/wp-includes/option.php#L42

That means that you have to manipulate the return data in order to be sure of the type of data that you are dealing with.

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