Защита данных очень важна.

Because защита данных means protection of data, and in Russian that of data relationship is usually shown with the genitive case.

• защита = protection
• данные = data
• данных = of data

So:

• защита данных = data protection / protection of data

If you used данные here, it would not fit the grammar of the phrase.

Защита is in the nominative singular.

It is the subject of the sentence:

• Защита данных = data protection
• очень важна = is very important

So the whole sentence is structured like:

• [Subject] + [predicate]
• Защита данных
  • очень важна

Важна is the short-form adjective of важный and it agrees with защита, which is feminine singular.

Compare:

• важный = masculine
• важная = feminine, full form
• важно = neuter short form / adverb in some contexts
• важна = feminine short form

In this sentence, Russian normally uses the short form after the subject to mean is important:

• Защита данных очень важна. = Data protection is very important.

Using важная here would usually sound incomplete unless it is followed by a noun:

• важная тема = an important topic

So:

• важна = predicative, is important
• важная = attributive, important before a noun

In Russian, the verb to be is usually omitted in the present tense.

So English:

• Data protection is very important

becomes Russian:

• Защита данных очень важна.

But in the past or future, Russian does use forms of быть:

• Защита данных была очень важна. = Data protection was very important.
• Защита данных будет очень важна. = Data protection will be very important.

Because the main noun, the grammatical head of the subject, is защита.

The phrase защита данных means:

• protection of data

The important word grammatically is защита, and данных only depends on it.

So the adjective must match защита, which is:

• feminine
• singular

That is why we get:

• важна

not a plural form.

In this sentence, данных is the genitive plural form of данные.

The word данные often behaves like a plural noun meaning data. In many contexts, Russian treats it as plural rather than as a mass singular noun.

So:

• данные = data
• данных = of data

That is why защита данных literally means protection of data.

Yes, Russian word order is fairly flexible, though some versions sound more natural than others.

The neutral order here is:

• Защита данных очень важна.

You could also say:

• Очень важна защита данных.

This puts more emphasis on very important.

In normal writing, though, Защита данных очень важна is the most straightforward and neutral version.

Очень is an adverb meaning very. It modifies важна:

• важна = important
• очень важна = very important

It works much like English very, and it usually comes right before the adjective or adverb it modifies.

No. Russian has no articles.

So Russian does not distinguish grammatically between:

• data protection is very important
• the data protection is very important

The exact interpretation depends on context.

That is normal in Russian, and learners should not try to insert a word for the or a.

A careful pronunciation with stress marks is:

• Защи́та да́нных о́чень важна́.

Approximate pronunciation:

• za-SHEE-ta DAN-nykh O-chen' vazh-NA

A few useful notes:

• щ in защита sounds like a soft long sh-type sound
• х in данных is like the ch in Scottish loch
• the stress in важна is on the last syllable: важна́

Sometimes yes, depending on context, but not always exactly.

• Защита данных most directly means data protection
• In some contexts, especially IT or policy language, English may also say data security, but that is not always identical in meaning

So if you are learning the grammar of this sentence, it is best to understand:

• защита = protection
• данных = of data

and therefore:

• защита данных = protection of data / data protection

It is a complete sentence.

Why?

Because it has:

• a subject: Защита данных
• a predicate: очень важна

Even though there is no written word for is, Russian does not need it in the present tense. So this is a full sentence, not just a noun phrase.