From the official documentation
The purpose of the bundlelinks tool is to reduce the number of links in a dataset by merging (or bundling) adjacent links together. By merging links, you can distill a visually complex representation into one which effectively summarizes the link structure of your data. A bundle is encoded as a new link with a single start and end position, which are formed by the boundaries of the merged links. Bundles are best shown using the ribbon feature.