EDIT (explanation): There are lots of things a Moving Average tells you. On smaller time frames it can provide potential support, resistance and when paired with faster or slower moving averages at higher time frames, it can become a very effective signal. A crossover occurs when a faster Moving Average (i.e. a shorter Moving Average period of time) crosses either above a slower Moving Average (considered a and depicted above in green) or below it, which is considered a crossover (depicted above in red). Taken just on it's own, it can be difficult to time your trade entry with an MA cross as by it's very nature, it employs lagging indicators. The MA cross above uses 9 & 21 to look at the moving average of the last 9 and the last 21 daily candles at 'close'. When paired with a companion indicator (like RMI in this instance) you can more effectively time your entry with confirmation at the plotting of the crossover. The sell signal was at the break below 80 of the RMI; the blue plot of the crossover (circled in red) tells us we should stay short until the next cross is plotted.
Higher time frame trades like this can be painful if you're not prepared for large swings and though it has been successful in every instance going back at least 1 year (5 other times) there's always a first time for it to fail. Lastly, it is just as important to properly time your exit, as it is for your entry.