What is Trend following, time-series momentum?

Trend following is trading a market in the direction of its own recent price move, long after rises and short after falls, across many markets at once. Time-series momentum is its simplest rule: the position in each market has the sign of that market’s own past excess return over a lookback of $L$ days, whatever other markets did; cross-sectional momentum (chapter 5) instead ranks markets against each other.