What flex-width/height does?

flex-width stands for flexible width and flex-height stands for flexible height

    <?php
    // Previous function code here
    $args = array(
            'default-image'      => get_template_directory_uri() . '/assets/images/default-image.jpg',
            'default-text-color' => '000',
            'width'              => 1140,
            'height'             => 250,
            'flex-width'         => false, // Now width are fixed to 1140 px
            'flex-height'        => true,  // Height is adjustable
        );
    // Later Code Here
    ?>