"MRD" is an abbreviation for "Marketing Requirements Document".
An MRD is a description of a new product that doesn't exist yet. It's a little bit like a spec (short for "specification"), but it's not so specific. The ones I've seen have been long lists of things that the product "MUST" do/have, "SHOULD" do/have, or "MAY" do/have, interspersed with things that "MUST NOT" be so. The capital letters appear like that: "The back panel MUST provide IRIG input blah blah blah" and so on. It doesn't go into terribly fine detail about the implementation details. It says, "give me something so I can truthfully tell a prospect that we've got this list of features."
The name "Marketing Requirements Document" is accurate: It's a description of what the marketing and sales people believe a product must be in order for people to buy it. Very few products (and even fewer successful ones) exist because they "ought" to exist, or because somebody was inspired or enlightened. They exist because somebody will buy them: Somebody wants to make a living selling things, and you can't sell things without things to sell. This is not a joke. It's very real and quite serious. There are people out there trying to think of things that you might like to buy, but that nobody's making yet. There are a lot of guys out there in business casual clothes sitting in each others' offices saying things like this: "Jim, god damn it, if we could ship a box with dual wingy flugles on the back at a lower price point, we'd have that Throckmorton account all sewed up and we could piss all over Lemming Industries in Q4."