It allows for pulling whatever is saving in the options table in the column _content1 to be added as content into the header,footer, etc of the WordPress theme.
Whoever injected the code most likely set some html to _content1 option, like a viagra add, or they have a another script on the server waiting to update the option. You probably want to look for strange .php files in the root of your site. There may be a simple eval($_REQEST) waiting for you.