This is a common trick to produce abstract colorful waves in illustrator. First you draw a curve line using the Pen Tool and drag a copy of it by holding Alt/Option. Select the Blend Tool and click the first line and hold ALt/Option while you click the second line to show the Blend Options. Change Spacing to Specified Steps and set it to 5.
Select the Direct selection Tool and edit the curve. Make sure the 2 lines intersect each other to crate a nice wave. Change one of the line stroke to a darker color to see it blend into a colorful wave.
This technique can be used to create nice colorful waves for background deign. Try blending 3 lines of different color to make it even more attractive.