Polylines are continuous lines composed of multiple segments. To create them, click the Polyline button from your drawings toolbar, to the left of the Text box button.
After drawing a segment of your polyline, click to end the segment and begin your next segment. If you want to cancel the last segment and go back to selection mode, press Escape on your keyboard. When you've finished, connect the endpoint of your last line segment with the origin point of your polyline to create an enclosed shape. Alternatively, click near the endpoint of your last line segment to complete the shape.
When you're done drawing the polyline, you remain in polyline mode so you can immediately draw another one. Press Escape to return to selection mode.
If you'd like to rearrange the vertices, double-click the polyline.