The custard will present a certain impedance (resistance and reactance, normally represented as a complex number) at a frequency. A radio transceiver normally expects 50ohms (real) at the antenna port for maximum energy transfer (and smallest standing wave voltage ratio). Using a network of capacitors and inductors (the tuner) the custard's impedance can be transformed to 50 ohms. But it's still not an efficient antenna, in the sense that it won't radiate more.
You are using the ubiquitous jargon of radio engineering. But for those on this site who are not radio engineers: of course the impedance of the custard does not change. The impedance of the system including the custard and the antenna tuner is different – and better for the radio to work with — than the custard alone.
I mean i don't understand it. He puts some wire into custard and then a device is 'tuning' and then the reception is better after?