OnEdit

function onEdit(e){
  const sheetname = 'Data';
  const column = 3;
  const startRow = 2;

  if( e.range.getRow() >= startRow &&
      e.range.getColumn() == column &&
      e.source.getActiveSheet().getName() == sheetname &&
      Number(e.value) == 1){
        const offsetValue = e.range.offset(0, -1).getValue();

        GmailApp.sendEmail('RemcoE33@example.com', `${offsetValue} is running low!`, '');
  }
}