Jeg forstår uttrykket.

Because Norwegian usually marks the definite form by adding an ending to the noun.

• et uttrykk = an expression
• uttrykket = the expression

Here, uttrykk is a neuter noun, so the definite singular ending is -et.

Compare:

• et ord → ordet
• et uttrykk → uttrykket

So uttrykket already means the expression all by itself.

It is built into uttrykket.

In English, you usually use a separate word:

• the expression

In Norwegian, you often attach the definite ending directly to the noun:

• uttrykket

So Jeg forstår uttrykket literally looks something like:

• I understand expression-the

That is completely normal in Norwegian.

Because uttrykk is a neuter noun in Norwegian.

Norwegian nouns have grammatical gender. In Bokmål, the main possibilities are:

• en for many masculine nouns
• ei for many feminine nouns
• et for neuter nouns

Since uttrykk is neuter, its indefinite form is:

• et uttrykk

and its definite singular form is:

• uttrykket

Unfortunately, noun gender is something you often just have to learn together with the noun.

Forstår is the present tense of å forstå (to understand).

So:

• å forstå = to understand
• forstår = understand / understands / am understanding depending on context

A very useful thing to remember is that Norwegian verbs do not change according to the subject the way English verbs do.

So you get:

• jeg forstår = I understand
• du forstår = you understand
• han forstår = he understands
• vi forstår = we understand

The verb form stays the same.

Because forstår is a finite verb in the sentence, not an infinitive.

• å forstå = to understand
• jeg forstår = I understand

You use å with the infinitive form, like English to:

• Jeg vil forstå uttrykket. = I want to understand the expression.

But in a normal present-tense sentence, you do not use å:

• Jeg forstår uttrykket.

You add ikke (not):

• Jeg forstår ikke uttrykket.

In a normal main clause, ikke usually comes after the finite verb:

• Jeg
  • forstår
    • ikke
      • uttrykket

So the basic pattern is:

• subject + verb + ikke
  • object

For a yes/no question, Norwegian usually puts the verb first:

• Forstår jeg uttrykket? = Do I understand the expression?

This is the usual verb-subject inversion pattern.

Compare:

• Jeg forstår uttrykket. = statement
• Forstår jeg uttrykket? = yes/no question

A more natural everyday example might be:

• Forstår du uttrykket? = Do you understand the expression?

Yes, very often.

Both forstå and skjønne can mean understand.

• Jeg forstår uttrykket.
• Jeg skjønner uttrykket.

In many situations, skjønner sounds a bit more everyday and conversational, like I get it or I understand.
Forstår can sound slightly more neutral or formal.

But in a sentence like this, both are perfectly natural.

In one common standard pronunciation, jeg sounds roughly like yai.

Important points:

• the j sounds like English y
• the spelling does not sound like English jeg

So jeg is not pronounced like jegg.

Also, pronunciation varies a lot by dialect. You may hear forms like:

• jeg
• je
• eg

But in written Bokmål, jeg is the standard form.

Not always.

Uttrykk can mean several related things depending on context, such as:

• an expression
• a phrase
• an idiom
• a way of wording something
• sometimes even a more general expression in the sense of style or appearance

So the exact meaning depends on context. In a learning sentence like this, it often means something like expression or phrase.

Because with a simple definite noun, Norwegian normally uses just the noun with its definite ending:

• uttrykket = the expression

You would not normally say:

• det uttrykket
  unless you mean that expression or you are adding special emphasis.

So:

• Jeg forstår uttrykket. = I understand the expression.
• Jeg forstår det uttrykket. = more like I understand that expression

There is one important extra point: if you add an adjective, Norwegian often uses both a separate determiner and the definite ending:

• det vanskelige uttrykket = the difficult expression

But in your original sentence, no separate article is needed.