Those are references, and they are similar to “pointers” in C or C++.
More information is in the PHP manual.
In this case, since
$db_hosts is empty, the construct
$db_hosts[$db_item['hostid']] will create a new array with an empty item on the index of
$db_item['hostid'] and return the item’s reference, making
$db_host act as an ‘alias’ for whatever
$db_hosts[$db_item['hostid']] is currently.