It’s kind of like writing something on paper, then erasing the pencil mark, then writing new material. Every time you do this, the writing and erasing part physically removes some of the paper each time. On the SSD’s it’s digital, so the high electrical voltage actually burns 1’s and O’s to the material. In a SSD, there is an electrical erasing and rewriting process. I think the actual data material in the SSD is some sort of metal oxide like graphite.
I’m not a Mac person, but there should be either a utility pre-installed or you can find one that will access the health of your SSD.