What is passed into SIGNAL/SLOT function parameters?

[Plissken]

What is passed into SIGNAL/SLOT function parameters?

connect( lineEdit, SIGNAL( textChanged(const QString &) ),
             this, SLOT( enableFindButton(const QString &) ) );
 
void FindDialog::enableFindButton( const QString &text )
{
    findButton->setEnabled( !text.isEmpty() );
}

connect( lineEdit, SIGNAL( textChanged(const QString &) ),
             this, SLOT( enableFindButton(const QString &) ) );

void FindDialog::enableFindButton( const QString &text )
{
    findButton->setEnabled( !text.isEmpty() );
}

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enableFindButton is called (by who exactly? The operating system?) but what is passed into const QString &? Obviously it's either going to be an empty string or not but what value does it hold? Do most if not all signals and slots of your own creation have to have a parameter?