Гидратация теста слишком высокая.

Because гидратация теста means the hydration of the dough.

In Russian, when one noun describes another with an of relationship, the second noun is often put in the genitive case:

• гидратация теста = hydration of dough
• цвет хлеба = color of the bread
• запах кофе = smell of coffee

The base form is тесто. Its genitive singular form is теста.

In Russian, the verb to be is usually omitted in the present tense.

So:

• Гидратация теста слишком высокая. literally: Hydration of dough too high natural English: The dough hydration is too high.

This is completely normal in Russian.
If you wanted past or future, then forms of to be would appear:

• Гидратация теста была слишком высокой. = The dough hydration was too high.
• Гидратация теста будет слишком высокой. = The dough hydration will be too high.

Because it agrees with гидратация, which is a feminine noun.

Russian adjectives must agree with the noun they describe in:

• gender
• number
• case

Here:

• гидратация = feminine, singular, nominative
• so the adjective must also be feminine, singular, nominative: высокая

Compare:

• уровень слишком высокий = the level is too high
  (уровень is masculine)
• влажность слишком высокая = the moisture is too high
  (влажность is feminine)

Because it is part of the predicate describing the subject.

The subject is:

• Гидратация теста = the dough hydration

The sentence says something about that subject:

• it is too high

In Russian present-tense equational/descriptive sentences like this, the descriptive adjective is commonly in the nominative:

• Цена высокая. = The price is high.
• Температура нормальная. = The temperature is normal.
• Гидратация теста слишком высокая. = The dough hydration is too high.

Yes. That is also correct.

Russian has two forms here:

• высокая = full adjective form
• высока = short adjective form

So both are possible:

• Гидратация теста слишком высокая.
• Гидратация теста слишком высока.

The difference is mostly stylistic:

• высокая often sounds more neutral and conversational
• высока can sound a bit more formal, concise, or written

In modern everyday speech, the full form is often more common.

Слишком means too in the sense of more than is acceptable / more than needed.

So:

• слишком высокая = too high

This is different from simply saying very high:

• очень высокая = very high
• слишком высокая = too high

So слишком implies a problem or excess.

This is the most neutral word order for this sentence.

Structure:

• Гидратация теста = subject
• слишком высокая = predicate description

Russian word order is more flexible than English, so other orders are possible, especially for emphasis:

• Слишком высокая гидратация теста.
• Гидратация теста высокая слишком. (possible, but less neutral and more marked)
• Слишком высока гидратация теста.

But the original version is the most straightforward and natural if you are simply stating a fact.

No. Here it means high in the sense of a high level/value.

Russian uses высокий / высокая / высокое not only for physical height, but also for abstract measurements:

• высокая температура = high temperature
• высокая влажность = high humidity
• высокий уровень = high level
• высокая гидратация = high hydration

So this is just like English.

It is a real Russian word, but in this context it sounds somewhat technical or specialized, especially in baking.

Among bakers, гидратация теста is normal terminology. It refers to the ratio of water to flour.

Outside that context, many people might more naturally talk about:

• тесто слишком влажное = the dough is too wet
• в тесте слишком много воды = there is too much water in the dough

So the sentence is natural in a baking or recipe discussion.

Russian has no articles.

So Russian nouns do not directly mark the difference between:

• a dough hydration
• the dough hydration

That information comes from context instead.

In this sentence, English will usually use the because the context probably refers to a specific dough or recipe:

• The dough hydration is too high.

But Russian simply says:

• Гидратация теста слишком высокая.

Here теста is the genitive singular of тесто.

That can be confusing, because some Russian endings can look similar across forms. But here the meaning and the noun pattern make it clear:

• nominative singular: тесто
• genitive singular: теста

So гидратация теста means hydration of the dough, not hydration of doughs.

Also, тесто is usually treated as an uncountable mass noun in this kind of context, much like dough in English.

A helpful stress guide is:

• гидратАция тЕста слИшком высОкая

More precisely:

• гидратАция
• тЕста
• слИшком
• высОкая

If you are reading aloud, the main intonation usually falls near the end, on высОкая, since that is the key descriptive information.