A single moving average shows the trend; two of different lengths can signal when it changes. A crossover system uses a fast MA and a slow MA, and treats the moment they cross as a trend signal — a simple, mechanical way to define direction.
Golden and death crosses
When the fast MA crosses above the slow MA, momentum has turned up — a bullish signal, famously called a golden cross when it is the 50 crossing above the 200. When the fast crosses below the slow, it is a death cross — bearish. The fast line reacting first is what makes the cross a (lagging) trend-change alert.
The lag problem
Because both lines are averages of past prices, a crossover always confirms a move that has already begun — you get in (and out) late. In a clean trend that is fine; in a choppy range it is a disaster, as the lines cross back and forth producing a string of losing whipsaw signals. Crossovers reward patience and punish ranges.
Using them well
Crossovers are best as a trend filter rather than a precise entry: only take long setups while the fast MA is above the slow, for example, and let other tools (structure, levels, oscillators) time the actual entry. Combining a slow-moving trend filter with a faster timing tool is the core idea behind most trend-following systems.