Double bottoms/double tops are a chart pattern formation as shown in the above picture. The pattern to the left shows a double bottom. The formation of a double bottom shows a strong support level at that point. You can place a buy order just above the 2nd bottom. The same concept applies to the double tops.
The chart above shows a double-top formation. As you can see there is strong resistance at the double top. If you placed a trade (sell order) just after the second top you could collect some pips.
The chart below shows the formation of a double bottom followed by a bullish engulfing pattern. You could place a trade after the formation of the 2nd bottom or the bullish engulfing pattern.