The following code is from `export_stockinfo_xls` in Odoo 10. I want to know what mean `context.get('active_ids', [])` and what is the returned value. Why do we use `[]` in that expression?

```
@api.multi
def export_xls(self):
    context = self._context
    datas = {'ids': context.get('active_ids', [])} # what mean this and what return

    datas['model'] = 'product.product'
    datas['form'] = self.read()[0]

    for field in datas['form'].keys():
        if isinstance(datas['form'][field], tuple):
            datas['form'][field] = datas['form'][field][0]
    if context.get('xls_export'):
        return {
            'type': 'ir.actions.report.xml',
            'report_name': 'export_stockinfo_xls.stock_report_xls.xlsx',
            'datas': datas,
            'name': 'Current Stock'
        }

```