Nie stresuj się, wynik testu nie jest najważniejszy.

Because stresować się is the normal verb here, not just stresować.

• stresować kogoś = to stress someone out
• stresować się = to get stressed, to worry, to stress out

So się is an essential part of the verb in this sentence. Without się, you would usually be talking about causing stress to someone else, not about the person’s own emotional reaction.

Stresuj is the 2nd person singular imperative form of stresować się.

It is the form used when speaking directly to one person:

• stresuj się = stress yourself / get stressed
• nie stresuj się = don’t get stressed

Related forms:

• Nie stresujcie się = talking to more than one person
• Niech pan/pani się nie stresuje = polite/formal singular

So this sentence is addressed informally to one person.

In this kind of imperative sentence, verb + się is the normal order:

• Nie stresuj się

For learners, that is the safest pattern to remember.
Polish word order is flexible in some situations, but się is a clitic-like word and does not usually go at the very beginning of a clause. In an imperative like this, putting it after the verb is the most natural choice.

Because Polish often expresses of-relationships by putting the second noun in the genitive case.

So:

• wynik = result
• testu = of the test

That means testu is the genitive singular form of test.

This is a very common pattern in Polish:

• koniec filmu = the end of the film
• kolor domu = the color of the house
• wynik testu = the result of the test

Yes, wynik z testu is also possible and very common in everyday speech.

There is a small nuance:

• wynik testu = test result, a bit more compact/neutral
• wynik z testu = result from the test, very natural in conversation

In many situations, both work. The version in your sentence, wynik testu, sounds neat and standard.

Because in Polish, nie is written separately with finite verb forms.

So:

• jest = is
• nie jest = is not

This is different from many adjectives, where nie is often written together:

• ważny = important
• nieważny = unimportant

In your sentence, the negation is on the verb jest, not on the adjective najważniejszy, so it is written as nie jest.

Najważniejszy is the superlative form of ważny.

The pattern is:

• ważny = important
• ważniejszy = more important
• najważniejszy = most important

So naj- is the prefix that creates the superlative.

Because it agrees with wynik, which is a masculine singular noun.

In Polish, adjectives must agree with the noun they describe or with the subject they refer to. Here:

• wynik is masculine singular
• so the adjective is najważniejszy

Compare:

• ocena nie jest najważniejsza — ocena is feminine
• zadanie nie jest najważniejsze — zadanie is neuter

So the ending is determined by wynik, not by the speaker.

In a normal sentence like this, jest should stay.

• Wynik testu nie jest najważniejszy = standard Polish

If you remove jest, the result sounds very elliptical, poetic, or like a headline, not like ordinary speech.

So unlike some languages that often omit to be in the present tense, standard Polish keeps it in this structure.

The comma separates two closely connected clauses:

• Nie stresuj się
• wynik testu nie jest najważniejszy

This is natural in Polish when one clause is followed by another explanatory or supporting clause, even without a word like because.

You could also write them as two separate sentences:

• Nie stresuj się. Wynik testu nie jest najważniejszy.

That would sound a little more separated and emphatic. The version with a comma feels smooth and conversational.

Yes, Polish word order is fairly flexible, but the version you have is the most neutral:

• Wynik testu nie jest najważniejszy.

Other orders are possible, but they change emphasis:

• Najważniejszy nie jest wynik testu — stronger focus on najważniejszy
• Nie jest najważniejszy wynik testu — more marked, less neutral

For a learner, the sentence as given is the best standard pattern to remember.