Questo dato è la base della relazione.

Because dato is a masculine singular noun, so the demonstrative must agree with it.

• questo = masculine singular
• questa = feminine singular

So:

• questo dato = this piece of data
• questa relazione = this report / this relationship

Italian words usually have to agree in gender and number with the nouns they describe.

Here, dato means data point, piece of information, or fact/data depending on context.

A useful thing for English speakers to notice is that Italian often uses dato as a singular countable noun, while in English data is often treated as a mass noun.

So:

• un dato = a piece of data / a datum
• due dati = two data points / two pieces of data

In this sentence, questo dato is most naturally this piece of data or this datum.

The accent distinguishes è from e.

• è = is
• e = and

So in this sentence:

• Questo dato è la base... = This datum is the basis...
• without the accent, it would be a different word entirely

This is a very common spelling point in Italian.

Italian often uses the definite article where English might not.

So è la base literally means is the base, but in natural English that is often translated as:

• is the basis
• is the foundation

Saying Questo dato è base della relazione would sound unnatural or incomplete in standard Italian. The article la is normally needed here.

Della is the contraction of:

• di = of
• la = the

So:

• di + la = della

Because relazione is a feminine singular noun, la is the article it takes, and after di they combine into della.

So:

• la relazione = the report / the relationship
• della relazione = of the report / of the relationship

Because relazione is feminine singular.

The contracted forms of di + article must match the gender and number of the noun:

• del = di + il for masculine singular
• della = di + la for feminine singular

Since it is:

• la relazione

you get:

• della relazione

not del relazione.

Relazione can mean different things depending on context, including:

• report
• relation
• relationship
• sometimes connection

In a sentence like Questo dato è la base della relazione, the most likely meaning in many formal or academic contexts is:

• This piece of data is the basis of the report

But if the broader context is about links or connections between things, it could also mean:

• the relation
• the relationship

So the exact English translation depends on the situation.

No, Italian word order is somewhat flexible, though the original sentence is neutral and natural:

• Questo dato è la base della relazione.

You could also say:

• La base della relazione è questo dato.

That version puts more emphasis on la base della relazione first, as if answering the question What is the basis of the report?

So both are possible, but they do not feel exactly the same in emphasis.

Yes. That is a very useful thing to notice.

As a noun:

• un dato = a piece of data / a datum

As a past participle of dare:

• dato = given

For example:

• Ho dato il libro a Marco. = I gave the book to Marco.
• dato il problema, dobbiamo trovare una soluzione = given the problem, we must find a solution

In your sentence, dato is clearly a noun, because it is preceded by questo and functions as the subject.

A very literal translation is:

• This datum is the base of the report/relation.

But more natural English would often be:

• This piece of data is the basis of the report.
• This data point is the basis of the report.
• This fact is the foundation of the report.

So even if the Italian uses base, English often prefers basis in this kind of sentence.