Market timing is when people try to figure out when to buy or sell their investments based on what they think will happen in the future. Imagine you are playing a game and you want to be the winner. You might try to predict what move your opponent will make and then make your own move based on that prediction. In the same way, people try to predict what will happen in the stock market and then buy or sell their stocks to make money.
But it is really hard to predict what will happen in the future. Even grown-ups who have studied the stock market for years don't always get it right. It's like trying to guess what the weather will be like next week - sometimes you might be right and sometimes you might be wrong.
One important thing to remember is that when you buy a stock, you own a tiny piece of a company. If the company does well, the value of the stock may go up and you can sell it for a profit. But if the company does badly, the value of the stock may go down and you could lose money.
So, market timing can be risky because you might sell your stocks when the market is low thinking it will go even lower, but then it might suddenly go up and you will miss out on the higher value. Or you might buy stocks thinking they will go up, but then something unexpected could happen and the stock could go down.
Even experienced investors can't time the market perfectly every time. The best way to make money in the stock market is to invest for the long-term and hold onto your stocks even when the market goes up and down.