It's the price correcting when it went way too high, it happens because the bid and demand are not balanced

Bitcoin correction is a temporary decline in price interrupting a usual uptrend of bitcoin price. This usually happens after a very good run and investors are cashing-out their profit. Corrections also happens when bitcoin comes to a point that people no longer wants to buy due to very high price, bitcoin looses demand which triggers price drop. But in any market, corrections are considered healthy as it promotes good opportunities for trading and investments.

If you are specifically asking about the price correction, then to put it in simpler terms this is the price wherein btc does not have a sudden pump or dump in value. You would see that in a particular period, lets say a week or a month you would not see too much volatility and there is a strong support and resistance on the charts.

A "correction" is a general market term for when the price sells off quite quickly, usually after a substantial rise.  You do not know when this will occur, or how far down it will fall, its called in retrospect.