Wir halten die Außeneinheit frei.

On its own, halten often does mean to hold, to keep, or to stop depending on context. But in this sentence, frei halten is a fixed expression meaning:

• to keep clear
• to keep unobstructed
• to leave free

So Wir halten die Außeneinheit frei means something like We keep the outdoor unit clear.

This is a very common pattern in German:

• den Eingang freihalten = to keep the entrance clear
• die Tür freihalten = to keep the door unobstructed
• den Fluchtweg freihalten = to keep the escape route clear

So it is not about physically holding the unit in your hands.

Because the full verb is really freihalten, which is a separable verb.

• infinitive: freihalten
• main clause: Wir halten die Außeneinheit frei.

In a normal main clause, the conjugated part goes in position 2, and the separable part goes to the end.

Compare:

• Ich halte die Tür offen.
• Wir halten den Bereich sauber.
• Sie hält den Weg frei.

So halten is the conjugated verb, and frei is the separated part/complement placed at the end.

In the dictionary form, it is usually written as freihalten.

But when it is conjugated in a main clause, it often appears split:

• Wir halten die Außeneinheit frei.
• Bitte haltet den Eingang frei.

So both ideas are connected:

• dictionary form: freihalten
• finite main-clause form: halten ... frei

That is completely normal for separable verbs in German.

Here, die Außeneinheit is in the accusative case because it is the direct object of halten.

The sentence structure is:

• Wir = subject
• halten = verb
• die Außeneinheit = direct object
• frei = predicative complement / separable element

Since Außeneinheit is a feminine noun, the article is:

• nominative: die
• accusative: die

So the form die does not change here, but the function is still accusative.

Because frei is not being used directly before a noun here.

German adjectives usually take endings when they come before a noun:

• die freie Außeneinheit = the free/clear outdoor unit

But in your sentence, frei is used predicatively, after the verb phrase:

• Wir halten die Außeneinheit frei.

Predicative adjectives in German do not take adjective endings.

Compare:

• Die Außeneinheit ist frei.
• Wir halten die Außeneinheit frei.

In both cases, frei stays plain.

Außeneinheit is a compound noun:

• außen = outside, external
• Einheit = unit

So Außeneinheit means outdoor unit or external unit, often in a technical context, such as:

• an air-conditioning unit
• a heat pump unit
• another device installed outside a building

German very often builds technical vocabulary by combining nouns like this.

The ß is called Eszett or scharfes S.

In modern German spelling, ß is often used after a long vowel or diphthong. In außen, the au is a diphthong, so you get ß:

• außen
• Außeneinheit

If you cannot type ß, writing ss is often understood in informal contexts, but the correct standard spelling is Außeneinheit.

A rough English-style guide would be:

• AUSS-en-EIN-hite

More precisely:

• Auß- sounds like owss
• -en- is a short unstressed syllable
• Ein- sounds like eye-n
• -heit sounds a bit like hite

The main stress is usually on the first part of the compound: AUßeneinheit.

Yes. The German present tense often covers both ideas:

• We keep the outdoor unit clear
• We are keeping the outdoor unit clear

German does not need a separate progressive form like English are keeping. The exact meaning depends on context.

So Wir halten die Außeneinheit frei could describe:

• a general rule or policy
• a habitual action
• what is happening right now

You could, but it means something a bit different.

• freihalten = to keep clear / leave unobstructed
• frei machen = to make free / clear away

So:

• Wir halten die Außeneinheit frei.
  = We keep it clear.

• Wir machen die Außeneinheit frei.
  = We are clearing it now / making it unobstructed.

The first one focuses more on the ongoing state or rule.
The second one focuses more on the action of clearing it.

Yes, very often. This is a very useful pattern in German.

Examples:

• Wir halten den Eingang frei. = We keep the entrance clear.
• Bitte halten Sie die Tür frei. = Please keep the door clear.
• Haltet den Fluchtweg frei. = Keep the escape route clear.
• Sie halten den Bereich sauber. = They keep the area clean.

So the structure is often:

subject + halten + object + adjective/complement

And the meaning is usually keep something in a certain state.