The Blend tool
Draw 2 lines that cross each other
Start by drawing 2 curving lines using the pen tool. Make sure these 2 lines cross each other at least once. This will create this rather cool effect once we apply the Blend tool. Give the lines a stroke color.
Apply the blend
Select both lines using the Selection Tool (black arrow). Now double click the Blend tool in the Toolbox to get the Blend Options box. Enter 20 for the amount of steps and select the 2nd orientation option. Click OK. Now with the tool and both lines still selected, click in the starting point of the first line and then click in the starting point of the second line.
Edit the path or color if needed
You should get a blending effect of different lines in between as shown in the above image. With the Direct Selection tool you can select one of the lines you've originally drawn. The lines in between are not visible in outline mode. Yu can't select or edit them, but you can still modify the color of both lines you've drawn and you can change the shape or even add points if you like.
You can use the Blend tool on any shape. You can blend from a star to a circle, a line to a rectangle, etc. If I'm my memory serves me right I use to create gradients like this using this same technique back in older Illustrator versions. You can still do this but Illustrator offers you far more easier ways and the Blend tool is more likely used for special effects like for instance this line effect described here.
Distort Zig Zag Effect
Another way to achieve interesting line effects starting from a simple shape, is by using the Distort Zig Zag Effect. I'm choosing the Zig Zag effect because this effect will give a rather symmetrical end result compared to the other Distort effects. They can be interesting to experiment with if you're after a more random or even chaotic end result. Also one advice if I may, choose Effects over Filters if possible. Effects are adjustable at any time using the Appearance palette. Filters on the other hand don't give you that flexibility and freedom. Once applied, you need to use the undo to change the result and start over. So keep that in mind ;)
Draw a circle (hold down shift key). Give the circle a stroke or line filling. Go to the Effects menu and choose Zig Zag from the Distort submenu. Check the preview option and play with the sliders.
Or draw a star (using the Star tool, hidden under the Rectangle tool). I chose to give the star a dashed line. Go to the same Effect again and play with the sliders. Again, don't forget to check the preview option to see the end result.
My end result, just lines
