هذه تعليمات مهمة.

هذه is the demonstrative meaning this or these.

In this sentence, it means these, because it points to تعليمات (instructions). Arabic uses هذه with non-human plural nouns, even though هذه is grammatically feminine singular.

So the structure is basically:

هذه = these
تعليمات مهمة = important instructions

Together: These are important instructions.

This is one of the most important agreement rules in Arabic:

Non-human plural nouns are usually treated as feminine singular for agreement.

Since تعليمات means instructions and is non-human, Arabic uses the feminine singular demonstrative هذه instead of a true plural demonstrative.

That is why you get:

هذه تعليمات مهمة

not a plural demonstrative form.

This same rule also affects adjectives and verbs.

For the same reason: تعليمات is a non-human plural, and in Modern Standard Arabic, non-human plurals usually take feminine singular agreement.

So:

• تعليمات = plural noun
• مهمة = feminine singular adjective

This may feel strange to an English speaker, but it is completely normal in Arabic.

Compare the logic:

• non-human plural → feminine singular agreement
• human plural → real plural agreement

So مهمة is exactly what you expect here.

Arabic normally does not use a present-tense verb meaning to be in simple sentences like this.

So English says:

These are important instructions.

But Arabic simply says:

هذه تعليمات مهمة

There is no separate word for are in the present tense.

This kind of sentence is called a nominal sentence. The meaning of is/are is understood automatically.

Because this sentence is not saying these instructions are important. It is saying these are important instructions.

That difference matters in Arabic.

Here, هذه is the subject, and تعليمات مهمة is an indefinite predicate:

• تعليمات = instructions
• مهمة = important

So the whole predicate means important instructions.

If you said:

هذه التعليمات مهمة

that would mean:

These instructions are important.

And if you said:

هذه التعليمات المهمة

that would usually be understood as a noun phrase:

these important instructions

not a complete sentence in the same way.

تعليمات is a plural noun meaning instructions.

Its singular form is تعليمة, meaning an instruction.

So:

• تعليمة = one instruction
• تعليمات = instructions

Because it ends in ـات, it is a sound feminine plural in form.

This is a nominal sentence in Arabic.

A simple way to analyze it is:

• هذه = subject
• تعليمات مهمة = predicate

Inside the predicate:

• تعليمات is the main noun
• مهمة is an adjective describing it

So the sentence literally has the structure:

These + important instructions

which naturally becomes in English:

These are important instructions.

A fully vocalized version would be:

هٰذِهِ تَعْلِيمَاتٌ مُهِمَّةٌ

The ending ـٌ on تعليماتٌ and مهمةٌ shows that they are indefinite and in the nominative case.

In normal everyday writing, these short vowels are usually omitted, so you usually see:

هذه تعليمات مهمة

Yes, but it means something slightly different.

• هذه تعليمات مهمة = These are important instructions
• هذه التعليمات مهمة = These instructions are important

So the sentence you were given uses an indefinite predicate: important instructions.

The version with التعليمات makes instructions definite: the instructions / these instructions.

Both are correct, but they do not mean exactly the same thing.

It happens with many non-human plurals in Modern Standard Arabic, not just تعليمات.

For example:

• هذه كتب جديدة = These are new books
• هذه سيارات سريعة = These are fast cars

Notice that the adjective is feminine singular:

• جديدة
• سريعة

So هذه تعليمات مهمة follows a very common Arabic pattern.