In a DC circuit, a capacitor is equivalent to an open circuit. A capacitor is a device that can store charges and is also one of the most commonly used electronic components.
This starts with the structure of the capacitor. The simplest capacitor is composed of two end plates and an insulating dielectric (including air) in the middle. After being electrified, the electrode plate is charged, forming a voltage (potential difference), but due to the insulating material in the middle, the entire capacitor is non-conductive. However, such a situation occurs without exceeding the critical voltage (breakdown voltage) of the capacitor. We know that any substance is relatively insulated. When the voltage at both ends of the substance increases to a certain extent, the substance can conduct electricity, and we call this voltage the breakdown voltage. Capacitors are no exception. After a capacitor is broken down, it is no longer an insulator. However, in middle school, such voltage is not visible in circuits, so it works below the breakdown voltage and can be seen as an insulator.
However, in AC circuits, the direction of current changes as a function of time. The process of charging and discharging capacitors takes time, and at this time, a changing electric field is formed between the electrode plates, which is also a function of time. In fact, current flows through capacitors in the form of a field.
In middle school, there is a saying that is called "open AC, resistance DC", which refers to the property of capacitance.