What does a roller coaster relationship mean?

My current crush had a bad break up. They were in a relationship for 1 and half years. They were dating and it seemed to be fine. when he graduated, they broke up because he moved out. They got back together 5 months later and he was insecure and could not find a job but after 4-6 months later they broke up again for good this time.
when I read what his ex wrote, she said it has been a rollercoaster. lt was filled with disappointment and hard these few months. Plus my crush always threatened to leave her.
When I met my crush, he was cold, pushed me away and mention that girl each time sometimes he was aggressive in his words although he was interested. So I dug deeper to understand him and found that 2 years post break up, he couldn't move on. I managed to get him out of survival mode and start living again. He speaks about me with many of his close friends which for me is good sign that he has finally moved on but I find it weird how he talks to me in indirect ways (is this part of roller coaster relationship?)
since I managed to make him move on, I am confident that slowly and surely I will be able to pull him closer. But can anyone explain to me what is a roller coaster relationship?
What does a roller coaster relationship mean?
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