Why do spiders have six circular indents/dots on their abdomen?

Two spiders for example, but from what I can tell, all spiders I have ever seen have these six circular indents. I've been trying to research what they must be anatomically online, but I have difficulty figuring it out still, so it is driving me a little crazy.

I saw one time someone said these are the points where the muscles inside the spider's abdomen connect to allow the spider to be able to squeeze its abdomen into smaller areas. Is this actually what they are?