Je is the 3rd person singular of biti = to be, and in Croatian it behaves like a clitic. Clitics usually come in the second position in the sentence or clause.
So in:
the first element is Oštećenje, and je comes right after it.
This second-position behavior is very common in Croatian.
With measurements after adjectives like dug (long), širok (wide), visok (high/tall), Croatian commonly uses a measure expression without a preposition.
So you get patterns like:
In this sentence, jedan metar is the measurement phrase. Also, with metar, the singular nominative and accusative look the same, so metar does not change form here.
Compare:
Not always.
Croatian often omits jedan in measurement expressions when English would still say one:
Both are possible.
Using jedan can sound a bit more explicit, exact, or contrastive.
Yes, you can also say Oštećenje je jedan metar dugo, and many learners will hear that order very often.
Croatian word order is fairly flexible, especially with measurement phrases. So both of these work:
The second version may sound a bit more neutral or common to many speakers, but the original sentence is understandable and grammatical.
Yes, you may hear dugačak / dugačka / dugačko as well.
So a similar sentence could be:
But with exact measurements, dug is very common and standard:
So dugo in your sentence is perfectly normal.
In a sentence like this, oštećenje usually refers to a specific damaged area, defect, or damaged section, not just the abstract idea of damage in general.
That is why it can be described with a physical measurement like one meter long.
So the sentence is talking about the extent of the damaged part.
Because Croatian does not have articles like a/an and the.
So oštećenje can mean:
The exact interpretation depends on context.
That is normal in Croatian, and learners have to get used to reading definiteness from the situation rather than from an article.