Symmetrical triangles are patterns where the slope of the price’s highs and the slope of the price’s lows converge together to a point where it looks like a triangle. What is happening during this formation is that the market is making lower highs and higher lows. This means that neither the buyers nor the sellers are pushing the price far enough to make a clear trend. If this was a battle between the buyers and sellers, then this would be a draw.
Neither the buyers nor the sellers could push the price in their direction. When this happens we get lower highs and higher lows. As these two slopes get closer to each other, it means that a breakout is getting near. We don’t know what direction the breakout will be, but we do know that the market will break out. Eventually, one side of the market will give in.
This type of activity is called consolidation.