%s is a C format specifier for a string.
msg = g_strdup_printf (_("<b><big>Enter your password to run " "the application '%s' as user %s" "</big></b>"), command, context->user);
means “where you see the first %s
, replace it with the contents of command
as a string, and where you see the second %s
, replace it with the contents of context->user
as a string.