The bookmark system is a network of devices spread throughout our section of the galaxy. Their function is to assist in the creation of portals between stars. A portal if formed by folding space until two points are close enough together to make a bridge. We see this as a hole in space. The further you have to bend the space the larger the distance you can travel but also the more energy you have to use. For interstellar distances this energy requirement becomes too heavy, making those sorts of distances unfeasible.
The Bookmark Devices help with this by making permanent folds wherever they are located. A single Bookmark consists of a ring of solar collectors orbiting around a star. The star provides them with the massive amount of energy they need in order to function. The rings power a massive space-fold generator. These generators are always operating and holding the same fold in space at any given moment. Though the fold is always active the device does not form a portal. It can only be found and then used if you already possess portal technology. Essentially it takes the fold and then punch a hole process of portals down to simply punching the hole.
Most bookmarks cover a distance of about 10 to 50 lightyears. The Bookmark device connecting two stars together only needs to be around one of the stars. As such, the system was built so that no planets of value had the bookmark around their own star but rather around the neighboring star they were connected to instead. This also greatly reduced the odds of a species visually seeing the bookmark, even with telescopes.