Power hour is a term used in the stock market, and it refers to the closing hours of stock exchanges which is from 3 pm till 4 pm American time.
3 pm - 4 pm is regarded as the power hour because there is more buying/selling that takes place as market participants struggle to close their positions & open overnight positions before the stock exchanges close for the day.
Remember that investors/traders usually pay special attention to the closing price of an asset so once the stock exchange is about to close market participants will try to estimate what the possible closing price of an asset will be and execute overnight orders to take advantage of the possible closing price.
Another reason why the power hour is important is that most economic news/data is released in the afternoon so investors/traders will take an hour or two to digest this news then open trades based on this news during the power hour.