Hello Tabea,
This is a option where fibers are created such that a structure with non-touching fibers is made. In that option the fibers are placed in the structure and when they violate the given isolation distance to another fiber they are removed and a new fiber is placed and checked for violation. This happens until the stopping criterion is reached (for example a given Solid Volume Percentage or a given numbre of fibers).
But in general this approach can take much time for high solid volume fractions so in general we recommen to use the "Remove Overlap" option, here you can also set an Isolation distance.