The "trend line" is the green line on the weight graph.  To show a trend, you have to log your weight frequently (but never more than once a day!)  The trend is the general direction in which your weight is moving.  It is there to smooth out the day-to-day fluctuations in your actual weight.  When the trend line goes down, you are on a weight loss trajectory; when it is flat, you have reached a plateau; when your trend line goes up, you are gaining weight - even if you have lost weight on a particular day.