ON_BN_CLICKED(IDC_BUTTON1, &CSetDlg::OnBnClickedButton1)

This code is a message handler for when the user clicks on a button with the ID "IDC_BUTTON1". When the button is clicked, the function specified in this message handler will be executed. The specific function that will be executed depends on the code that is written within the curly braces following this message handler declaration.