From Traderpedia

In Technical Analysis, a line that is drawn across the top or bottom of price action, connecting the highs and lows on a chart.

Trendlines that slope upwards are bullish, and trendlines that slope downwards are bearish.

The theory behind trendlines is to give an indication that the trend of an instrument is changing. For example, if price breaks through an upward-sloping trendline, analysts might consider that to be of bearish sentiment, and an indication that the current price may not be sustainable.