Underscores within echo statements

Just a quick question. What is the purpose of the underscore character within the following echo statement? I had a quick look in the manual and archives but couldn't find any reference to it. <?php echo _("Username") ?> Thanks, Neil This communication is confidential to the intended recipient(s). If you are not that person you are not permitted to make use of the information and you are requested to notify the sender immediately of its receipt then destroy the copy in your possession. Any views or opinions expressed are those of the originator and may not represent those of Advanced System Architectures Ltd. *** This Email Has Been Virus Checked ***