Shortcodes in WordPress are bits of text you can use in the content area to invoke some kind of function to accomplish certain tasks. Например, video embedding in WP 2.9+ uses the shortcode. You can write your own shortcodes, and plugins often offer their functionality via shortcodes as well.
But what if you want to use a shortcode from within a template instead of with the content of a Post/Page? You can invoke it with a special function:
<?php echo do_shortcode("[shortcode]"); ?>