There are numerous cyclical themes in Dark. It is the defining motif across the seasons.
Predominantly, it is used to refer to the fact that the events we are viewing are happening over and over. The predominant cause of that are Adam and Eve, who intentionally kill Martha/Jonas to set Jonas/Martha on a path that causes them to later become Adam/Eve; on top of many other actions they take to ensure that the cycle repeats.
Adam may not realize that his actions cause the cycle to repeat, but they effectively do. Eve, on the other hand, is actively working to repeat the cycle, because it's the only way for the Unknown to exist.
There are other cyclical events, such as Charlotte and Elisabeth's parentage, but those are not as essential to the core plot.
Note that Jonas and Martha's parentage is also cyclical, though it takes a few more steps:
- Jonas > Unknown > Tronte > Ulrich > Mikkel/Michael > Jonas
- Martha > Unknown > Tronte > Ulrich > Martha
There are many more characters who are caught up in this cyclical parentage, or who could only have been born because of time travel (e.g. Silja, since Egon and Hannah are from different times).
All of these characters are notably missing from the final dinner scene in the finale, because they only existed in Adam and Eve's worlds as their existence relies on either the cycle or time travel in general.