It’s a browser extension/add-on (or as Tumblr like to scare-tactic call it, a “browser hack”) that adds functionality to Tumblr in many wonderful ways. Tumblr is truly cheesed off that people are using a product that makes their site more intuitive and easy to use, thus spending more time on the site, and a few months ago started making you click a “scary” pop-up about how Missing E is terrible and will harvest your personal data and eat your kittens and whatever. Problem is, the pop-up continues to appear even after you check the “I understand you’re being ridiculous” box, AND it pops up when you’re logged into your account from a computer or device that doesn’t have Missing E installed (like from an iPhone or Android phone, which aren’t even capable of having such things installed).
So what that means is that TUMBLR ITSELF is collecting data about your browser extensions (like, I have Adblocker and Greasemonkey and a bunch of other things, too), and associating them with your account and using that information against you. Which seems to be exactly what they warn you Missing E will do (but never ever has).
Cutlerish is the creator of Missing E. He seems to be an awesome guy, he responds to users in a timely and professional manner, he keeps up with all of Tumblr’s pointless and stupid changes, and they continue to threaten him for creating a product that makes their product actually usable.
I am incapable of making a long story short. Sorry.