An ideal diode is a diode that acts like a perfect conductor when voltage is applied forward direction (forward biased). It also pretends like a perfect insulator when voltage is applied reverse direction (reverse biased). So when positive voltage terminal is applied to the anode and negative to the cathode, the diode conducts forward current instantly.
The Idea Diode is like an one way road. You can ride/drive in one way only. (Just legally unless you are the law breaker… ). The ideal diode allows the current to flow from its anode(+) to the cathode(-).
Forward Bias & Reverse Bias
When a voltage is applied in reverse bias, that is when the performs no current at all. This diode operates like a switch. When the diode is in forward bias, it works like a closed switch. Whereas, if an ideal diode is in reverse bias, then it works like an open switch.
In Forward Bias, the currents flows in the Circuit as shown. The circuit is complete in this case, the diode allows the flow of current from the positive terminal of the battery to the cathode of the diode and then back to the cell. Please note the terminals will be flipped in the next case below.
In Reverse Bias, the current flows only to the anode of the diode. In this case, the diode acts like an open switch. There is no further path for the current to flow hence diodes are used in applications where flipped polarities can be harmful in an electronic or electrical devices.