Polarity is the direction of current flow in a circuit from the two poles, positive and negative. You require two points of differential charge for any electron movement to take place.
In an AC circuit, each pole alternates between positive and negative 60 times per second. You can see from the image of the AC sine wave, that the change in polarity of AC does not happen instantly, although close to and 60 times per second (Hertz).
(Top positive side of the sine wave) Voltage increases from 0V to its max “peak” voltage, which is 120V in the above example. At that point, the voltage then begins to decrease back to 0V.
(Bottom negative side of the sine wave) The voltage then reverses polarity and drops below 0V, again heading for the peak voltage but the negative polarity.
In a DC circuit, it’s between the negative and positive with current flowing from negative to positive.