These are the Drag and Anti-Drag wires

One of the big strength qualities of any structure is triangulation.  

The drag and anti-drag wires provide this strength at a low weight and in a small package.  If you think of the forces that are placed on a wing you will notice that there is a pressure applied during lift, an opposite force due to landing and gravity when the plane is sitting still, a force due to the drag during flight, and a force that pushes the wings forward when the plane hits something which is the opposite of drag or anti-drag.  These forces of drag and anti-drag are countered by the drag and anti-drag wires.  Even though it's not technically correct, builders most commonly refer to both of these sets wires as drag wires. 

The red rubber tubing that is placed between these wires prevents chafing of the wires as the engine and other vibrations create oscillations within these wires.  If this chafing was allowed to occur, the wires would suffer a severe loss of strength and eventual wing failure would result.  The use of rubber has a secondary effect of dampening harmonic oscillations within the wires which result as a function of the wire material, length and tension applied to it. This can be compared to a guitar string which oscillates at a given tone and can be dampened when you place your finger lightly on the string.   Not shown here, but in a completed wing the crossing point of these wires are tightly tied using lacing cord or other strong material.

There's one more very important thing that must be added to this subject.  When the holes for the drag and anti-drag wires are drilled, they are not drilled exactly on the center line of the spars.  They are drilled so that one wire is above the spar centerline and the other wire is below the centerline. This is to allow a vertical spacing between the wires where they cross.  You absolutely need this spacing!  If the wires touched each other or are ever so slightly bent around each other, even with the rubber tubing, they will eventually chafe and lead to a reduction of the strength of the wing.