The most commonly-cited example is the 50 MA crossing above or below the 200 MA. Usually expressed in days. So if 50 DMA crosses below the 200 DMA, you get "death crosses". OOOhhhh! Halloweeen time! Insert any numbers you want for MAs. They are all arbitrary numbers.
Quick entry and exit on FB, small loss. RSI looked as oversold as I've ever seen. Crosses over trendline. Did I ignore the need for a trigger candle again, and if so, are there any options besides bullish engulfing candles I should be waiting for in that situation? Think I keep confusing retracement with reversal.
There were 13 consecutive bear bars prior to that bull entry under your falling moving average(s), no strong break of the trendline, and there wasn't a clear exhaustion bar with a break of the trend channel line. Don't fight the trend.
Context is more important than one basic rule. As a beginner, you're going to want that at an absolute minimum, but you're going to lose a lot more than you win just going long at a first breakout attempt. Wait for the test of the extreme and a good signal bar before going long; you should really wait for confirmation too, but where's the fun in that. Test it with your risk management and see if it makes sense for you to wait for confirmation or not.