La rotación de cultivos protege la fertilidad del suelo.

La is the definite article meaning the. It agrees with rotación, which is a feminine singular noun.

• la rotación = the rotation
• not el rotación, because rotación is feminine

In Spanish, it is very common to use the definite article in general statements like this one.

Rotación is a noun, while rotar is a verb.

• rotación = rotation
• rotar = to rotate

In this sentence, the subject is a thing/concept: crop rotation. So Spanish uses the noun phrase:

• La rotación de cultivos = Crop rotation

If you used rotar, you would need a different structure with a verb.

De cultivos means of crops.

So:

• rotación = rotation
• de cultivos = of crops

Together, rotación de cultivos is the normal way to say crop rotation.

Spanish often uses noun + de + noun where English may use a noun directly before another noun:

• rotación de cultivos = crop rotation
• casa de campo = country house
• zumo de naranja = orange juice

Because crop rotation normally involves different crops, not just one crop.

• cultivo = crop / cultivation
• cultivos = crops

So rotación de cultivos is the standard expression. Using the singular would sound unusual here unless you were talking about a very specific case.

Protege is the third-person singular present form of proteger (to protect).

The subject is La rotación de cultivos, which is singular, so the verb must also be singular:

• La rotación de cultivos protege... = Crop rotation protects...

Conjugation of proteger in the present:

• yo protejo
• tú proteges
• él/ella/usted protege
• nosotros protegemos
• vosotros protegéis
• ellos/ellas/ustedes protegen

Because fertilidad is a feminine singular noun.

• la fertilidad = the fertility

Many Spanish nouns ending in -dad are feminine:

• la ciudad
• la posibilidad
• la calidad
• la fertilidad

Because de + el contracts to del.

So:

• de + el = del

That is why you get:

• la fertilidad del suelo = the fertility of the soil

This contraction is required in normal Spanish.
A similar contraction happens with a + el = al.

In this context, suelo is the best word for soil, especially in agriculture or science.

• suelo = soil, ground, land surface
• tierra = earth, land, soil, dirt

Both can sometimes relate to soil, but suelo sounds more precise and technical here. In a farming or environmental sentence, la fertilidad del suelo is the most natural phrasing.

Yes. Protege is in the present simple.

Here it expresses a general truth or fact:

• La rotación de cultivos protege la fertilidad del suelo.
• Crop rotation protects soil fertility.

Spanish uses the present tense very often for facts, habits, and general statements, just like English.

Yes, but it means something slightly different in tone.

• La rotación de cultivos = crop rotation in general
• La rotación de los cultivos = the rotation of the crops / rotation of specific crops

Without los, the phrase sounds more general and is the usual way to name the concept. With los, it can sound more specific, as if you are talking about particular crops already known from the context.

The normal order here is:

subject + verb + object

• La rotación de cultivos = subject
• protege = verb
• la fertilidad del suelo = object

This is the most neutral and natural order.

You can change the order in Spanish for emphasis, but it may sound less neutral:

• La fertilidad del suelo la protege la rotación de cultivos.

This is grammatical, but more marked and not the simplest version for a learner.

A rough pronunciation guide:

• rotación → ro-ta-THYON in most of Spain
  (in much of Latin America: ro-ta-SYON)
• protege → pro-TE-he
  The g before e sounds like a strong Spanish h.
• suelo → SWE-lo

Stress:

• rotación
• protege
• suelo

Also note:

• the ción ending is very common and stressed on the last syllable before -n
• ue in suelo is a diphthong, so it stays together

Yes. It is very useful to learn rotación de cultivos as a set phrase meaning crop rotation.

This helps because English and Spanish build the phrase differently:

• English: crop rotation
• Spanish: rotación de cultivos

Learning it as one unit will make it easier to remember and use naturally.