No metas la sopa en el congelador hasta que esté menos caliente.

Because this is a negative command addressed to tú: Don't put...

In Spanish, negative tú commands use the present subjunctive, not the normal present tense.

• meter = to put / to place in
• tú metes = you put
• no metas = don't put

Compare:

• Mete la sopa... = Put the soup...
• No metas la sopa... = Don't put the soup...

So metas is the correct command form here.

Because hasta que often takes the subjunctive when it refers to something that has not happened yet.

Here, the soup is still hot, and the speaker is talking about a future point:

• hasta que esté menos caliente = until it is less hot

Since that state is still pending, Spanish uses the subjunctive:

• esté rather than está

This is very common:

• Espera hasta que llegue. = Wait until he/she arrives.
• No salgas hasta que termine. = Don't go out until it finishes / until I finish.

It means until.

The whole structure:

• No metas la sopa en el congelador hasta que esté menos caliente.

means:

• Don't put the soup in the freezer until it's less hot.

So the action of putting it in the freezer should happen only after the soup has cooled down somewhat.

Menos caliente literally means less hot. It does not necessarily mean cold.

That nuance matters:

• menos caliente = cooler than it is now
• fría = cold
• más fría = colder / cooler

So this sentence suggests:

• the soup does not need to become cold
• it just needs to cool down a bit first

A native speaker might also say:

• hasta que se enfríe un poco = until it cools down a bit
• hasta que no esté tan caliente = until it isn't so hot

But menos caliente is perfectly understandable and grammatically fine.

Because with meter, Spanish normally uses en to show that something is being put inside something.

• meter algo en algo = to put something into something

So:

• meter la sopa en el congelador = to put the soup in/into the freezer

Using al congelador would sound less natural with meter.
With other verbs, a/al can be normal:

• llevar la sopa al congelador = to take the soup to the freezer

But with meter, en is the expected preposition.

Spanish often uses the definite article where English would not.

Here la sopa means the soup, referring to the specific soup being discussed.

• No metas la sopa... = Don't put the soup...
• No metas sopa... would sound more like don't put soup in a more general or less specific sense

So la is there because the speaker has a particular soup in mind.

Yes. No pongas la sopa en el congelador... is also natural.

The difference is slight:

• poner = to put / place
• meter = to put in / stick into / place inside

With a freezer, meter feels very natural because it suggests putting something inside an enclosed space.

So both work, but meter is especially common in this kind of context.

Because Spanish often omits subject pronouns when they are understood from context.

In English, you need it:

• until it is less hot

In Spanish, the subject is understood to be la sopa, so you simply say:

• hasta que esté menos caliente

Spanish does this all the time:

• Espero que llegue pronto. = I hope he/she arrives soon.
• Cuando termine, te llamo. = When I/he/she/it finish(es), I'll call you.
  (The exact subject depends on context.)

Here, the meaning clearly points to the soup.

Yes, it agrees in principle, but caliente has the same form for masculine and feminine singular.

So:

• el café está caliente
• la sopa está caliente

Both use caliente.

Some adjectives change:

• frío / fría
• pequeño / pequeña

But others do not:

• caliente
• grande
• interesante

So la sopa esté menos caliente is perfectly normal.

Only the command form changes.

For tú:

• No metas la sopa en el congelador hasta que esté menos caliente.

For usted:

• No meta la sopa en el congelador hasta que esté menos caliente.

For vosotros (common in Spain):

• No metáis la sopa en el congelador hasta que esté menos caliente.

For ustedes:

• No metan la sopa en el congelador hasta que esté menos caliente.

Notice that esté stays the same, because it refers to la sopa, not to the person being addressed.