We have this spider on our back patio which has built an elaborate web and a distinctive zig zag in white. I think txrad told me that is to provide strength to the web. Here's what Wiki has to say:
The web of the yellow garden spider is distinctive: a circular shape up to 2 feet in diameter, with a dense zigzag of silk, known as a stabilimentum, in the center. The purpose of the stabilimentum is disputed. It is possible that it acts as camouflage for the spider lurking in the web's center, but it may also attract insect prey, or even warn birds of the presence of the otherwise difficult-to-see web. Only those spiders that are active during the day construct stabilimenta in their webs.