Left click (once) and drag to draw freehand. It will smooth the curve and place a manageable number of controls point along it.

Or you can single click/tap to place each point that the curve will pass through. Double click/tap to place the final point. Or right click to cancel placing the last point. You'll need to place at least three points to get a curve instead of a straight line.

You can also click using the Select and Move tool on a point to select it and move it to smooth out a line.