is the start of your while() loop. Basically the loop will run while the evaluation inside the brackets does not evaluate to false. The mysqli_fetch_assoc() function retrieves the next row from the result set specified in $result, but returns false when there are no more results to return. So, at the end of the results set, $row becomes false and the while loop ends.
Note that in this code, $result is not an array, it is an object which contains the results (or enough information for MySQL to provide the results) from your query. So you couldn’t var_dump($result) and see all the rows that your query returned. PDO has a function called fetchAll() which will return all rows as an array, and then you can iterate through the array as you wish.