In addition to what @Dec0y-jb said, It's also takes advantage of what PHP considers "truthy" and "falsey". To learn more about what PHP considers "true", see the truth table in the documentation.
http://php.net/manual/en/types.comparisons.php

Here, the code array_sum($a) % 2 returns an integer number. If it returns any number other than 0, the ternary operator sees it as a "true" condition, thereby returning the first value. However, if it returns exactly 0, the ternary operator sees it as a "false" condition, returning the second value.