Pročitaj poruku unatrag.

Pročitaj is the imperative form of the verb pročitati, meaning read in the sense of read through / finish reading.

So Pročitaj poruku unatrag. is a command to one person: Read the message backwards.

Because pročitati is perfective, it suggests a completed action: read it through in that way.

This is about aspect, which is very important in Croatian.

• čitati = imperfective = to read, be reading, read habitually
• pročitati = perfective = to read something through, to finish reading it

So:

• Čitaj poruku. = Read the message / Keep reading the message
• Pročitaj poruku. = Read the message (all the way through / completely)

In this sentence, pročitaj sounds natural because the speaker wants one complete action: read the message backwards.

It is the 2nd person singular imperative.

That means it is used when telling one person to do something:

• Pročitaj! = Read!

The verb comes from pročitati, and the imperative form for ti is pročitaj.

You would use the plural / polite imperative:

• Pročitajte poruku unatrag.

Croatian uses the same 2nd person plural form both for:

• speaking to several people
• speaking politely to one person

So:

• Pročitaj poruku unatrag. = to one person, informal
• Pročitajte poruku unatrag. = to several people, or to one person politely

Because poruku is the accusative singular form of poruka.

• poruka = message (dictionary form, nominative)
• poruku = message as a direct object

Here, the thing being read is the direct object, so Croatian uses the accusative:

• Pročitaj poruku. = Read the message.

This is very common in Croatian:

• Imam knjigu. = I have a book.
• Vidim kuću. = I see a house.
• Pročitaj poruku. = Read the message.

Unatrag means back, backwards, or in reverse.

In this sentence, it means:

• backwards
• from the end to the beginning
• in reverse order

So Pročitaj poruku unatrag. means to read the message starting from the end.

Here it is an adverb.

It describes how the action is done:

• read it how?
• unatrag = backwards

So the structure is:

• pročitaj = verb
• poruku = direct object
• unatrag = adverb of manner/direction

Yes, Croatian word order is fairly flexible.

The neutral order here is:

• Pročitaj poruku unatrag.

But other orders are possible for emphasis, for example:

• Poruku pročitaj unatrag.
• Unatrag pročitaj poruku.

These are not all equally neutral. The original sentence sounds the most straightforward and natural in most contexts.

Yes, but that is normal in Croatian.

The subject you is understood from the verb form:

• pročitaj already tells you the speaker is addressing one person

So Croatian usually does not need an explicit subject here.

You could say:

• Ti pročitaj poruku unatrag.

But ti is usually added only for emphasis, contrast, or clarification.

A rough English-style guide would be:

• Pročitaj ≈ pro-CHEE-tie
• poruku ≈ po-ROO-koo
• unatrag ≈ oo-NA-trag

A few useful notes:

• č sounds like ch in church
• j sounds like y in yes
• Croatian pronunciation is generally quite phonetic, so words are usually pronounced close to how they are written

It can mean both.

It can refer to:

• movement backward
• going back
• doing something in reverse order

So in Pročitaj poruku unatrag, it clearly means in reverse order / backwards, not physical movement.

Sometimes learners meet similar words such as unazad or naopako, but they are not exactly the same in every context.

• unatrag = backwards, back, in reverse
• unazad = also often means backward/back, and in many contexts overlaps with unatrag
• naopako = upside down, the wrong way round, incorrectly, awkwardly

For read the message backwards, unatrag is a good natural choice.

It is informal, because it uses the singular imperative pročitaj.

To make it formal or polite, use:

• Pročitajte poruku unatrag.

So the difference is not in the vocabulary, but in the verb form.