In php, you can set conditions for when to do something. For instance, is_search() in WordPress will be true anytime the page being displayed is a search page. To execute code when it is true you use brackets {}. These brackets can cover multiple lines and you can put normal ol’ html in between if you like. For instance:
<?php is_search() { ?>
<!-- notice no closing bracket -->
You're currently looking at a search page.
<?php } ?>
So all that inside those will only display when those conditions are true. If you’re interested (which I doubt, since that seemed just an idle question) there’s more info in the Codex.
And that’s where lonely brackets come from.
