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.
I could not have explained it any better than J.C. Hall, if you want to get into the nitty gritty you can click or tap here to see how to view more information regarding the S.M.A.R.T. status of your disk along with wear.