sikaeiseisi ga setumeisimasu.

が marks the grammatical subject of the sentence—the person who is doing the action.

So in:

歯科衛生士が説明します。

the 歯科衛生士 is the one who explains.

A natural breakdown is:

• 歯科衛生士 = dental hygienist
• が = subject marker
• 説明します = explains / will explain

So the structure is basically:

[The dental hygienist] [is the one who] explains.

This is a very common question.

Both が and は can appear with a noun before a verb, but they do different jobs:

• が marks the subject
• は marks the topic

With が, the sentence sounds more like it is identifying who does the explaining:

• 歯科衛生士が説明します。
  = The dental hygienist will explain. / It is the dental hygienist who will explain.

With は, the sentence would sound more like you are talking about the dental hygienist as a topic:

• 歯科衛生士は説明します。
  = As for the dental hygienist, they will explain.

In many situations both are possible, but が often feels more natural when introducing or specifying the person who does the action.

説明します is the polite non-past form, so it can mean:

• explains
• will explain

depending on context.

Examples:

• In a general statement: The dental hygienist explains.
• In an upcoming situation: The dental hygienist will explain.

It usually does not specifically mean is explaining right now.
For that, Japanese often uses a different form:

• 説明しています = is explaining

So 説明します is more like a neutral present/future.

Because 説明 by itself is a noun, meaning explanation.

To say to explain, Japanese often uses:

• 説明する = to explain

This is a very common pattern in Japanese:

• 勉強する = to study
• 運動する = to exercise
• 電話する = to call
• 説明する = to explain

Then the polite form of する is します, so:

• 説明する = explain
• 説明します = explain / will explain (polite)

Japanese does not have articles like English a/an and the.

So 歯科衛生士 can mean:

• a dental hygienist
• the dental hygienist

The exact meaning depends on context.

That means 歯科衛生士が説明します。 could be understood as:

• A dental hygienist will explain
• The dental hygienist will explain

English requires you to choose an article, but Japanese usually does not.

Because Japanese often omits information that is obvious from context.

In English, you might expect something like:

• The dental hygienist will explain the procedure.
• The dental hygienist will explain it to you.

But in Japanese, if everyone already knows what is being talked about, those parts can be left out.

So:

歯科衛生士が説明します。

can naturally mean something like:

• The dental hygienist will explain.
• The dental hygienist will explain it.
• The dental hygienist will give you an explanation.

Japanese relies heavily on context.

Yes. 歯科衛生士 is a single noun meaning dental hygienist.

It is made up of:

• 歯科 = dentistry / dental
• 衛生 = hygiene
• 士 = specialist / professional

So together it means a professional dental hygiene worker: dental hygienist.

Also, in normal Japanese writing, you would usually write the sentence without spaces:

歯科衛生士が説明します。

Spaces are often added in teaching materials just to make the parts easier to see.

It is pronounced:

しかえいせいし が せつめいします

A rough guide in English-style syllables:

• shi-ka-ei-sei-shi
• ga
• setsu-mei-shi-masu

A few notes:

• 歯科衛生士 = しかえいせいし
• 説明します = せつめいします
• Japanese rhythm is based on even beats, not English-style stress.

So try to say each part evenly rather than heavily stressing one syllable.

Yes. It is polite because of します, which is the ます-form of する.

• 説明する = plain / dictionary form
• 説明します = polite form

So:

• 歯科衛生士が説明する。 = plain, casual
• 歯科衛生士が説明します。 = polite

The polite form is very common in workplaces, clinics, customer service, and conversations with people you do not know well.

Yes, very often.

If it is already clear who will explain, Japanese might simply say:

説明します。

That can mean:

• I will explain.
• He/she will explain.
• They will explain.

depending on context.

In your sentence, the subject is included because it is useful to specify that the dental hygienist is the one who will explain.

No. Japanese nouns do not have grammatical gender like some European languages do.

So 歯科衛生士 just means dental hygienist, without saying male or female.

If the speaker wants to specify gender, they would need to add extra words, but normally Japanese does not do that unless it matters.