Trend indicators intended use is to identify the trend.
The most used trend indicator is the moving average.
You can, in most cases, find a link between a moving average and the security. An example is that the security signals demand pressure when the price reaches a moving average and supply pressure when it's extended.
The indicator determines the trend direction from the number of periods it is calculated on. If the price is above the moving average, the trend direction can be defined as up.
The choice of moving average length (periods it is calculated on) determines how sensitive the moving average is. The shorter length you use, the more sensitive and faster it is and reacts to new price moves.
There are a number of ways to calculate the moving average, but the two most common ones are simple and exponential moving average.
Additional popular trend following indicators includes MACD, Channel indicators, ADX, etc.