Horizontal lines are used to indicate support and resistance levels, psychological price levels or target for your take profit and Stop Loss levels.
In order to draw a horizontal line, go to the line studies toolbox, click the horizontal line button 
and then press anywhere in the chart where you would like it to appear. Once it is placed you can click it and move it around as you need.
Similarly, you can use vertical lines to mark an important period in your chart. To draw a vertical line, go to the line studies toolbox, click the 
button and then press anywhere in the chart where you would like it to appear.