How much background knowledge would I need to take advantage of it? I'm utterly ignorant of such matters, but I'm trying figure out how to go about learning this.
Unfortunately you will need some philosophical and historical background to fully comprehend the essay--Horkheimer writes, for instance, about the "Kantian Hope" which is a centerpiece of the short text--he also draws quite a bit on Nietzsche and some of his own previous work with Adorno. I'd say Kant and Nietzsche are the most important fellas to understand as far as comprehension of this essay is concerned. Chances are you'll still be able to get the gist of what Horkheimer is trying to say even if you don't have any background in the history of philosophy, but you'll definitely understand the essay better if you first read a summary of the developments and history of philosophy from about Plato to Nietzsche (essentially the major figures of philosophy up until 1900).
Alternatively, you can just dive in and look up what you don't understand as you read.
The Stanford encyclopedia of philosophy is a great resource for that sort of thing: https://plato.stanford.edu/
No problem. I'm also always looking to keep my philosophical muscles fresh, so if you ever want to chat about philosophy in general feel free to shoot me an email. My email is on my HN profile.
Alternatively, you can just dive in and look up what you don't understand as you read.
The Stanford encyclopedia of philosophy is a great resource for that sort of thing: https://plato.stanford.edu/