Sample Usage

function sample_admin_notice__success() {
    ?>
    <div class="notice notice-success is-dismissible">
        <p><?php _e( 'Done!', 'sample-text-domain' ); ?></p>
    </div>
    <?php
}
add_action( 'admin_notices', 'sample_admin_notice__success' );

The class notice-success will display the message with a white background and a green left border.

function sample_admin_notice__error() {
	$class = 'notice notice-error';
	$message = __( 'Irks! An error has occurred.', 'sample-text-domain' );

	printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) ); 
}
add_action( 'admin_notices', 'sample_admin_notice__error' );

The class notice-error will display the message with a white background and a red left border.

Use notice-warning for a yellow/orange, and notice-info for a blue left border.

The class name is-dismissible will automatically trigger a closing icon to be added to your message via JavaScript. Its behavior, however, applies only on the current screen. It will not prevent a message from re-appearing once the page re-loads, or another page is loaded.

[copied/pasted from https://codex.wordpress.org/Plugin_API/Action_Reference/admin_notices%5D