Signals and slots are used to connect between an event from gui and a function. In other words, you can manage situations what happens after an situations.

For example:

on this example, we say that, call the onButtonClicked() function after button clicked:

QObject::connect(ui.btnStart, SIGNAL(clicked()), this, SLOT(onButtonClicked()));

If you want to get signals, you must connect these to slots. Slots are functions defined as slot like this example: 

private slots:
          void onButtonClicked();

this code on header file.

And last important think is that, signals and slots must have same parameters. It works:

QObject::connect(ui.comboBox, SIGNAL(activated(int)), this, SLOT(onComboboxActivated(int)));

But there is no connection in this example:

QObject::connect(ui.comboBox, SIGNAL(activated(int)), this, SLOT(onComboboxActivated()));

QCombobox Signals And Slots