The codex says
shortcode_atts()
combines user shortcode attributes with known attributes and fills in defaults when needed. The result will contain every key from the known attributes, merged with values from shortcode attributes.
It doesn't make much sense to me (I'm a newbie).
Here is an example:
function wps_trend($atts) {
extract( shortcode_atts( array(
'w' => '500',
'h' => '330',
'q' => '',
'geo' => 'US',
), $atts));
$h = (int) $h;
$w = (int) $w;
$q = esc_attr($geo);
ob_start();
Please can you explain?