In my case, that means in part going through my inbox with a sharp eye and saying "can I unsubscribe from this?" I was astounded to figure out just how many times I was able to say "yes" to that question. The hope is that the inbox gets a little bit less cluttered.
I'm sure some marketing department got a little bit sad. Every unsubscribe routine is a little bit different, but the usual drill ends with a plaintive "we're sorry to see you go, did we do something wrong to offend you?" Either you ignore that screen, or you click the politest thing that makes sense to click. Sometimes there's a super-sad follow up automated email, "you're off the list now, did you make a mistake?"
No mistake. I don't need your weekly advertising specials, your cheap flights to places I'm not going to, your chirpy newsletter full of details of events I'm not going to. Trust me, if I need you, I can type your URL into a browser. What I don't need is a daily / weekly reminder that I'm not interested right now.