Jubelen var høy.

Because -en is the definite ending for many Norwegian nouns of common gender.

• jubel = cheering / jubilation / applause in a general sense
• jubelen = the cheering / the jubilation

So Jubelen var høy refers to a specific cheering sound, not cheering in general.

Var is the past tense of å være, which means to be.

• er = is / am / are
• var = was / were

So the sentence is in the past: The cheering was loud.

Here høy is a predicative adjective, meaning it comes after the verb var rather than directly before the noun.

In Norwegian, predicative adjectives still agree with the noun:

• common gender singular: høy
• neuter singular: høyt
• plural / definite contexts: often høye

Since jubelen is a singular common gender noun, høy is correct.

Compare:

• Jubelen var høy.
• Bråket var høyt. because bråket is neuter

Yes. Høy literally means high, but it is also used for loud when talking about sound level.

So in this sentence, høy means something like:

• loud
• strong
• at a high volume

This is similar to English expressions like high volume, although English usually says loud in normal speech.

Jubel is a noun. It refers to a strong positive reaction such as:

• cheering
• applause
• jubilation
• rejoicing

In this sentence, it most naturally refers to a loud cheering sound from a crowd.

This is the normal Norwegian word order for a simple statement:

subject + verb + adjective/complement

So:

• Jubelen = subject
• var = verb
• høy = complement

This works just like English The cheering was loud.

Yes, if the context already makes it clear what den refers to.

• Jubelen var høy. = full noun phrase
• Den var høy. = It was loud

But without context, Den var høy is less clear, because den could refer to many different common-gender nouns.

In modern Bokmål, jubel is normally treated as a common gender noun, and in practice that means it usually takes -en in the definite singular:

• en jubel
• jubelen

For a learner, the important thing is to remember that the sentence uses common-gender agreement:

• jubelen
• høy

The tricky part is øy, which is a very Norwegian vowel combination.

A rough guide:

• høy sounds somewhat like the vowel in French feuille or German euch
• for English speakers, there is no exact match

Also:

• h is pronounced
• y is not like English y
• the word is one syllable

If you already know øy from words like Norges øy or røyk, it is the same vowel combination.

Yes. Depending on context, English might translate it as:

• The cheering was loud.
• There was loud cheering.
• The applause was loud.
• The crowd was cheering loudly.

The exact best translation depends on what jubelen refers to in the wider context.