A trend is a price pattern, which is very important when trading in financial markets. Why ? Well, because getting into a trend enables you to 'ride' the trend and hence take a lot more money out of the market than by trying to second guess where a market price is going in volatile trading conditions. A trend is established when subsequent price 'bars' on a chart show either consecutive higher highs and higher lows, in the case of an uptrend, or lower highs and lower lows, in the case of a downtrend. The chart shows the FTSE in a downtrend, which, when broken indicates that the trend lower is coming to an end.
Uptrend and downtrend lines are used in combination with other technical indicators, such as moving averages and they can also be paired together to give price channels, which can then be employed to increase/decrease risk exposure within the trend.