Reading it they are reinforcing the wing skins, they will form the front D section of the wing, the flat side of the D being the spar, same thing they obviously carry some of the load and strength of the wing…. So it’s beefing it up.

It’s a major task because you need the skin to sit flush with the surface to ensure the strength is retained and you are working with compound curves (bend several ways at once, additionally you have to be uber accurate drilling out. And attaching the new skin, as you can weaken the structural strength with poor riveting / mis aligned holes.

A spar cap by the way think of a I beam the top and the bottom of the I are riveted on, these are the caps.