how do i fix this???

Hi all,

I’ve recently noticed that a lot of my clothing- shirts specifically- have these rings around the neckline of discolouration. It almost looks like a bleach stain, but I know it isn’t. I was originally thinking they were sweat stains, but I don’t really know anymore. It’s happened to so many shirts and it’s starting to upset me. I was wondering if anyone knows how to remove it so I don’t have to throw away the shirts.

The shirts that are most affected are a Target brand, if that’s important.

Any advice is much appreciated. Thank you.