It means that the permanent relationship, marriage, or partnership is "turbulent," like an old wooden roller coaster. There are ups and downs, and sharp turns left, then right, then rattles and rattles almost to a stop, but then the roller coaster cars take the connection below them and start up the hill. rattle and rattle all the way up until you climb to the top and roll slowly forward, then drop almost straight until level and turn left again.
This means that there are problems, problems, and things that were not foreseen from the beginning, that was not expected and with which both partners have difficulties. Above all, it "means" that the relationship may or may not last because it is so unstable, so bumpy and full of doubts and unexpected confusion and suffering that many couples simply "give up" because they feel that it is 'not worth the effort', or they feel that it is 'too difficult, that it is beyond their ability to 'weather the storm' and come out on the other side, exhausted, exhausted but wiser and tougher so we get through it together.
I hope my answer helps, as I have the feeling that a "turbulent" relationship is and is what it is. I've been to some of them myself and they can teach a person a lot about themselves and also who their partner is.