Drawing smooth curves

The statement as it stands is not really very helpful. Handles "as straight as possible in relation to each other" is what you get when you click and drag with the Pen Tool, or drag from an existing point with the Anchor Point Tool. The handles are then locked together such that they are tangent to the path at the point they share. That's as smooth as it gets. If the handles are unlocked from each other, the path forms a cusp, or corner at the shared point. The sharper the angle between the two handles, the "less smooth", I suppose, the path is, and the closer they are to sharing a straight line, the smoother the path.

I'm really not sure what that statement means, either.

Peter

Not only can you hold shift, as suggested above, if you show grids and turn on snap to grid, the handles will snap to the grid so that they stay maintain horizontal, vertical or diagonal and you will be able to keep control of their relative length by setting the grid settings.

What is your task? There are many tools that allow to draw smooth curves.

As reference I can suggest you

How to draw with the Pen, Curvature, or Pencil tool in Illustrator