O descanso melhora o condicionamento físico.

O is the masculine singular definite article, meaning the. In Portuguese, it is very common to use the definite article with general ideas and abstract nouns.

So o descanso is not necessarily talking about one specific rest period only. It can also mean rest as a general concept.

In English, we often leave out the article in general statements, but Portuguese usually keeps it:

• O descanso é importante.
• A saúde importa.

Because descanso is a masculine noun, so it takes o in the singular.

A very common pattern is:

• o for masculine singular nouns
• a for feminine singular nouns

Here:

• o descanso
• o condicionamento

Also, físico is masculine singular because it describes condicionamento, which is also masculine singular.

Descanso is a noun meaning rest, recovery, or sometimes a break, depending on context.

In this sentence, it most naturally refers to rest/recovery in a health or fitness sense. It is related to the verb descansar, which means to rest.

So this is not necessarily just sleep. It can include recovery time, not training all the time, and giving the body time to restore itself.

Melhora is the third-person singular present form of melhorar.

The subject is o descanso, which is singular, so the verb must also be singular:

• o descanso melhora
• not o descanso melhoram

In this sentence, melhorar is being used transitively, meaning to improve something.

So:

• O descanso melhora o condicionamento físico. = Rest improves physical conditioning / fitness.

Because the verb agrees with the subject, not with the object.

Here:

• Subject: o descanso → singular
• Object: o condicionamento físico

So the verb must be singular:

• o descanso melhora

If the subject were plural, then you would use melhoram:

• Os descansos melhoram...
  Though that exact sentence would be less natural in most contexts.

It is a noun phrase:

• condicionamento = conditioning
• físico = physical

Together, condicionamento físico usually means physical conditioning or fitness.

In Brazilian Portuguese, this expression is common in health, exercise, and sports contexts. It refers more to your body’s physical condition, training level, endurance, and overall fitness than to appearance alone.

Because in Portuguese, adjectives usually come after the noun.

So:

• condicionamento físico
• preparo físico
• atividade física

Putting físico before the noun would sound unnatural here. This expression is normally said exactly as condicionamento físico.

That o is the definite article for the direct object.

The structure is:

• O descanso = subject
• melhora = verb
• o condicionamento físico = direct object

Portuguese often uses definite articles in places where English may or may not use the. So even when English says something like improves physical fitness, Portuguese commonly says melhora o condicionamento físico.

It is in the simple present, but here it most naturally expresses a general truth or a general principle.

So the idea is:

• rest improves physical conditioning in general

Portuguese uses the present tense this way very often, just like English:

• A água ferve a 100 graus.
• O exercício ajuda a saúde.
• O descanso melhora o condicionamento físico.

Not in normal standard Portuguese. That sounds unnatural and incomplete.

In full, natural Portuguese, you would normally say:

• O descanso melhora o condicionamento físico.

Dropping the articles might happen in very special styles, like headlines, notes, or slogans, but not in ordinary speech or writing.

The tricky part for many English speakers is lh.

In melhora, lh makes a sound similar to the lli in some pronunciations of million, though it is not exactly the same.

A rough pronunciation guide is:

• me-LYO-ra

Other notes:

• físico has the stress on the first syllable: FÍ-si-co
• descanso is stressed on san: des-CAN-so
• condicionamento is stressed on men: con-di-ci-o-na-MEN-to

The original order is the most neutral and natural:

• O descanso melhora o condicionamento físico.

Portuguese word order is somewhat flexible, but you cannot freely move words inside fixed phrases like condicionamento físico.

You could make a different sentence with a slightly different focus, for example:

• O condicionamento físico melhora com o descanso.

That means something like Physical conditioning improves with rest. The meaning is close, but the structure is different.