Flat, or a sideways trend in the market is a situation where the price for some period of time does not grow and does not fall.
Price movement in a flat, as a rule, is short. Therefore, a flat is a signal to the trader that now is not the best time to make trades.
However, only 30% of the time the market is trendy and has a good strategy, you can make a profit during the other 70% of the time.
The best strategy of a flat is trading using support and resistance levels. If the price is approaching the support level top down and touches it, open an up trade. If the price approaches the support level bottom up and touches it, open a down trade.