What does double colons and underscore mean in Joomla?

What does double colons and underscore mean in Joomla?

I am learning basic PHP, trying to read and get some basic understanding
of JOOMLA core files, whereas I come across a lot of operators like "::_",
which I don't understand.
I did some google research and there are a lot of explanations regarding
double colons (::), but almost nothing on "::_", one of the files that
contain this operator is joomla-site-root/mod_login/tmpl/default.php, and
the line is
<form action="<?php echo JRoute::_('index.php', true,
$params->get('usesecure')); ?>" method="post" id="login-form">
So what does this mean? Is "JRoute::_('index.php', true,
$params->get('usesecure'))" a function call of class JRoute? if yes, what
function is it? I checked out the joomla
API(http://api.joomla.org/Joomla-Platform/Application/JRouter.html) but
still remain clueless. Please, any help would be greatly appreciated.