Realized volatility and implied volatility are both measures of how much the price of something (like a stock or a currency) can be expected to fluctuate in a given period of time.
Realized volatility is the actual amount that the price has moved over a particular period. You can think of it like taking a snapshot of how much the price of something moved.
Implied volatility is what traders expect the price to move over a certain period of time. It takes into account the amount that traders expect the price to move and is based on what people are buying and selling.
Think of it like taking a guess of how much the price will move in the future.