Sideways is a term that refers to a price trend that moves horizontally. The market moving like this is a period of stagnation after the asset has experienced an uptrend or downtrend. These three trends are price movements that exist in all asset markets.
In a sideways market trend, there will be an upper and lower limit range in the price. These are also commonly called support and resistance points. An asset that is moving horizontally usually moves between these ranges until it breaks out and rises above or falls to a lower price.
The sideways market is usually a good opportunity for traders to do swing trades because the price is moving within a clear price range. For investors, sideways moves can be very confusing. This is a fairly tedious period where the profits and losses are not very significant.