[du'u] is an obscure yet beautiful concept.
It's completely unnecessary, incidentally, for clearly translating this English sentence-- the elaborate phrasing in the English is for style, and should IMHO be translated with equally stylish Lojban, not an attempt at making Lojban as wordy as English. :) English phrases that simply establish the relationship of the sentence to fact or other propositions, like "it was true that," are often most easily translated by attitudinals (in this case perhaps [je'u]).
But since [du'u] is a beautiful concept, I'll try to explain my fuzzy understanding of it to you anyway. The abstraction created by [du'u] refers to the proposition enclosed, itself. It accepts a full bridi, and what it points to is the information contained in that bridi. It's something which can be true or false, for instance. Something which can be learned, remembered, believed in and understood.
By the very place structure of du'u, x1 is a predication (bridi) expressed in sentence x2, we can see that it's possible to put a du'u into a particular form, a "se du'u", but the du'u itself is different from the form. The du'u is the predication itself, as a relationship, and the se du'u is a particular way the predication is expressed, such as a Lojbanic bridi. Things begin to eat their own tails here, a bit, as the bridi which we've put into the du'u abstraction is itself a se du'u of the du'u, by definition, just not the only possible one! For instance, if you say "du'u mi citka lo plise kei" (the predication of me eating an apple), then one se du'u of that du'u we've described is "mi citka lo plise", as said, while another equivalent se du'u is "lo plise cu se citka mi".
Here's a typical use of du'u in a sentence: ".i mi krici le du'u le terdi cu bolci" I believe that the earth is round. Or, much more literally: I believe some predication, which I will describe to you thusly: It's the predication expressed in this bridi: "This thing called "earth" is a spherical object."
You can use a [du'u] phrase as a whole bridi. In that case you do not want to say "le", whose purpose is to turn a selbri into a sumti. If you put the "le", then you've just said one sumti and not said what relationship it's in-- a valid and common sentence form actually, but mostly used for answering questions. Without the "le", you can use a du'u as an observative bridi: "du'u le terdi cu bolci [kei]" What it means is "something (you probably can guess what) is the predication expressed by 'le terdi cu bolci'", which is rarely what you'd mean to say. Here's an artificial context that might make sense of it:
.i mi krici so'i du'u co'e
.i du'u le terdi cu bolci
.i du'u la .lojban. cu xamgu
(I believe many predications of something.. that the Earth is round, that Lojban is good.)
Thanks for asking. Lojban thrives on your curiosity.
mu'o mi'e .selkik.