In case you've been living under a rock, or you've been lucky enough to not be affected, here's the deal: Some bad guys, probably Russian or Eastern European, have decided to mail-bomb unsuspecting folks by signing them up for many hundreds or thousands of mailing lists. The bad guys built a tool that either searches for or has a list of signup forms at many hundreds or thousands of websites. The bad guys then submit many email addresses to those forms.
The net result is, if you're on the wrong end of this attack, your mailbox gets filled up with a bajillion newsletters. Some from big brands. Some from small brands. Some from companies you've heard of. Some from non-profits you've never heard of.