الغاية مهمة اليوم.

In Arabic, the present-tense verb to be is usually not stated in simple sentences like this.

So:

• الغاية مهمة literally looks like the goal important
• but it means the goal is important

This is completely normal in Modern Standard Arabic.

If you wanted a past meaning, you would usually add كان:

• كانت الغاية مهمة = the goal was important

It is a nominal sentence.

The parts are:

• الغاية = the subject/topic (مبتدأ)
• مهمة = the predicate/comment (خبر)
• اليوم = a time expression, today

So the basic structure is:

• subject + predicate + time expression

This is one of the most important things to notice.

In Arabic:

• الغاية مهمة = the goal is important
• الغاية المهمة = the important goal

Why?

Because when an adjective directly describes a noun inside a noun phrase, it must match the noun in definiteness:

• الغاية is definite
• so an attributive adjective would also have to be definite: المهمة

But here we have مهمة without الـ, so it is not attached as the important. Instead, it is the predicate: important.

Because الغاية is feminine, and the predicate adjective agrees with it in gender.

• masculine: مهم
• feminine: مهمة

Since الغاية is a feminine noun, Arabic uses the feminine form مهمة.

Here it is an adjective meaning important.

The word مهمة can also be a noun meaning task or mission, but the structure here strongly points to the adjective:

• الغاية مهمة = the goal is important

That is the natural reading.

اليوم means today, and here it gives the time setting for the whole sentence.

So the sense is:

• The goal is important today

It is functioning as an adverbial expression of time.

In fully vowelled Arabic, it is often analyzed as an accusative adverb of time, so you may see:

• اليومَ

Yes, اليوم can move.

The end position is very natural:

• الغاية مهمة اليوم

But Arabic can also shift it for emphasis or style:

• اليوم الغاية مهمة
• الغاية اليوم مهمة

All of these are possible, though they may sound slightly different in emphasis. The version you have is a very normal neutral order.

A careful fully vowelled reading is:

• الغايةُ مُهِمَّةٌ اليومَ
• al-ghāyatu muhimmatun al-yawma

In normal pause, many final case endings are not pronounced, so it may sound more like:

• al-ghāya muhimma al-yawm

A few pronunciation notes:

• غ in الغاية is a deep throaty sound, somewhat like a French-style r
• the final ة is pronounced like -a in pause, but as -t when case endings are fully pronounced, as in مهمّةٌ

Because غ is a moon letter, not a sun letter.

So the ل of الـ stays pronounced:

• الغاية = al-ghāya...

Compare that with a sun-letter word like الشمس, where the ل is not pronounced separately.

They are similar, but not always identical.

• الغاية often suggests goal, end, or ultimate purpose
• الهدف often suggests target or objective

In many contexts, both can be translated as goal, but الغاية can sound a bit more abstract or purpose-focused.

In fully formal, fully vowelled Arabic, the sentence is:

• الغايةُ — nominative, because it is the subject
• مهمةٌ — nominative, because it is the predicate of a nominal sentence
• اليومَ — accusative as an adverbial expression of time

So you get:

• الغايةُ مهمةٌ اليومَ

In most everyday reading and speaking, these endings are usually not pronounced fully, especially at pauses.