It's basically a "craft" or perhaps "vessel" - it could refer to a space ship, a boat, an aeroplane, a car or quite likely other means of transportation as well.
Those usually have more specific names, e.g. bIQ Duj "water craft" for a boat/ship, puH Duj "land craft" for a car, muD Duj "atmosphere craft" for an aeroplane, qam Do Duj "foot velocity craft" for a bicycle, and 'ejDo' "starship" (where the Do' part might be related to the modern word Duj historically, and the 'ej part is probably related to the 'ej in 'ejyo' "Starfleet", but neither 'ej nor Do' on their own are modern words having anything to do with stars or transportation).
In this course, Duj is usually translated generically as "ship", and I usually think of the prototypical Duj as a space ship of some kind.
Some older sentences might use "spaceship" or "starship" as the default translation of Duj, but that's not optimal and when we come across those we change them to "ship".
Using pronouns as verbs (like 'oH in your sentence) is reserved for equating two nouns and tIn is not a noun. tIn is a verb of quality (also sometimes referred to as a "be verb" or an "adjectival verb"). In English we can equate an adjective to a noun using the verb "to be" in the same way we can connect two nouns, but you can't do that in Klingon.
In Klingon, those verbs of quality already include the idea of "to be" (which is why they are sometimes called "be verbs"). Thus, putting a subject (like Duj) after them gives you a full sentence: tIn Duj "the ship is big".
Verbs of quality can also be used as adjectives (which is why they are sometimes referred to as "adjectival verbs"). But we can't put the verb of quality in front of the noun, like we do with adjectives in English, because then it looks like the noun is the subject of the verb. Instead, the verb of quality is placed after the noun to describe it in a noun phrase: Duj tIn "the big ship".
This is explained in the tips and notes for this unit -- I suggest that you go back and re-read them:
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