Hi,

active_id is the current record's id. You can get the active_id from the context as this:
active_id =  self.env.context.get('active_id')

The use cases are explained in this video in detail Active Id In Odoo Development

And for getting sale orders of the partner selected try this solution:

In your python file:

partner_id = fields.Many2one('res.partner', string='Customer')

def get_sale_orders(self):
    return {
        'name': _('Sale Orders'),
        'type': 'ir.actions.act_window',
        'res_model': 'sale.order',
        'view_mode': 'kanban,tree,form',
        'domain': [('partner_id', '=', self.partner_id.id)],
    }

In your xml file define the button(type=object) and partner_id field:

Pass the domain: 

'domain': [('partner_id', '=', self.partner_id.id)] , while returning from the button function

Thank you