La branca d'aquell arbre és massa baixa.

Because in Catalan, de often contracts or elides before a word beginning with a vowel.

• de + aquell → d'aquell

This is very common:

• l'arbre = el arbre becomes l'arbre
• d'aquell = de aquell becomes d'aquell

So La branca d'aquell arbre means the branch of that tree.

Because branca is a feminine singular noun in Catalan.

• la = feminine singular definite article
• branca = branch

So:

• la branca = the branch

Like many Romance languages, Catalan nouns have grammatical gender, and the article must match the noun.

Because adjectives in Catalan usually agree with the noun they describe in gender and number.

Here, branca is:

• feminine
• singular

So the adjective must also be:

• feminine
• singular

That gives:

• baix = low, masculine singular
• baixa = low, feminine singular
• baixos = low, masculine plural
• baixes = low, feminine plural

Since branca is feminine singular, we say baixa.

Massa means too.

So:

• massa baixa = too low

This is different from molt, which means very.

Compare:

• molt baixa = very low
• massa baixa = too low

That difference is important:

• very low is just a description
• too low suggests a problem or excess

In Catalan, words like massa normally go before the adjective they modify.

So:

• massa baixa = too low

This is similar to English:

• too low not
• low too

So the order is completely normal in Catalan.

Catalan, like Spanish, often distinguishes between ser and estar.

Here:

• és comes from ser
• està comes from estar

In this sentence, low is treated as a characteristic of the branch, not as a temporary condition, so ser is the natural choice:

• La branca ... és massa baixa.

A useful beginner rule is:

• ser = what something is like / a characteristic
• estar = a temporary state or condition

So és massa baixa means the branch is too low in a descriptive sense.

Aquell means that.

Catalan has a distinction similar to:

• aquest = this
• aquell = that

So:

• aquell arbre = that tree

Here aquell agrees with arbre, which is masculine singular.

Related forms are:

• aquesta = this, feminine singular
• aquella = that, feminine singular
• aquests / aquestes
• aquells / aquelles

Because arbre is a masculine singular noun.

So the demonstrative must match it:

• aquell arbre = that tree

Not:

• aquella arbre

Compare:

• aquell arbre = that tree
• aquella branca = that branch

The demonstrative changes depending on the gender and number of the noun.

Yes. Depending on context, baix/baixa can mean things like:

• low
• short or not tall in some contexts
• quiet/soft for sound in some expressions

But in La branca d'aquell arbre és massa baixa, the natural meaning is low.

So the sentence means that the branch hangs or sits at a low height.

This word order is normal and natural:

• La branca d'aquell arbre és massa baixa.

But Catalan can express a similar idea in other ways depending on emphasis. For example:

• Aquella branca és massa baixa. = That branch is too low.
• La branca és massa baixa. = The branch is too low.
  (if the tree is already understood)

So the original sentence is just one clear, standard way to say it.

It breaks down like this:

• La branca d'aquell arbre = the branch of that tree
• és = is
• massa baixa = too low

So the structure is:

[subject noun phrase] + [verb] + [adjective phrase]

More literally:

• The branch of that tree is too low.

This is a very common Catalan sentence pattern.

In standard Catalan, aquell is roughly pronounced like ah-KELL or uh-KELL, depending on dialect.

A few helpful points:

• the ll is traditionally a palatal sound, similar to the lli sound in some pronunciations of million
• in some modern dialects, it may sound closer to a y sound

You do not need perfect phonetics at first; the important thing is to recognize that aquell is the masculine singular form meaning that.

There is a word for of here: it is de, but it appears in the shortened form d'.

So:

• d'aquell arbre = of that tree

Catalan often uses de where English uses of:

• la porta de casa = the door of the house / the house door
• la branca d'aquell arbre = the branch of that tree

So the of idea is definitely present; it is just written as d' because of elision.