El atrezo está entre bastidores.

In Spain, atrezo refers to the props or set dressing used in theatre, film, or TV.

It usually means the objects placed on stage or on set to create the scene: furniture, decorative items, fake books, lamps, and so on.

A useful distinction is:

• atrezo = props / decorative objects
• decorado = the set / scenery itself

So atrezo is not usually the whole set, but the items used in it.

Because Spanish normally uses estar for physical location.

So when you say where something is, you usually use estar:

• El atrezo está entre bastidores.
• El libro está en la mesa.
• Los actores están en el escenario.

Using es here would sound wrong, because the sentence is about where the props are, not what they are.

Entre bastidores is an idiomatic theatre expression meaning backstage.

Literally, it is something like between the stage wings / behind the visible part of the stage.

In natural English, you would normally translate it as:

• backstage
• sometimes behind the scenes, depending on context

In this sentence, it clearly has the literal theatre meaning: the props are backstage.

Because the expression entre bastidores is a fixed phrase in Spanish.

The singular noun bastidor exists, but in this theatre expression Spanish normally uses the plural:

• entre bastidores

So it is best learned as a set expression, much like learning backstage as a single idea in English.

Because atrezo often works like a collective or mass noun.

This is similar to English words like furniture or equipment: one word, but it refers to multiple items.

So:

• El atrezo está entre bastidores. = the props / the set dressing is backstage

If you want to talk about individual items, you could say things like:

• los objetos de atrezo = the prop items
• los accesorios = the accessories

Yes, you could, and people would understand you.

But there is a nuance:

• entre bastidores = backstage, the standard theatrical expression
• detrás del escenario = behind the stage, more literal and less idiomatic in theatre language

So entre bastidores sounds more natural in a stage/performance context.

Spanish usually uses articles more often than English.

Here, el is used because atrezo is a masculine noun, and the sentence is talking about the specific props / the props in question.

Compare:

• El atrezo está entre bastidores. = The props are backstage.
• Hay atrezo entre bastidores. = There are props backstage.

The first refers to a known or specific set of props; the second just says that there are some props there.

It is especially natural in Spain, which matters here because you are learning Spanish from Spain.

In other Spanish-speaking countries, people may more often use other terms, especially utilería for props.

So for Spain:

• atrezo = very appropriate and natural in theatre/film/TV contexts

A simple pronunciation guide would be:

el a-TRE-tho es-TA en-tre bas-tee-THO-res

A few notes for Peninsular Spanish:

• z in atrezo sounds like th in think
• c before i/e would also have that sound in much of Spain
• the stress is on:
  • tre in atrezo
  • tá in está
  • do in bastidores

In IPA, roughly:

[el aˈtɾeθo esˈta ˈentɾe pastiˈðoɾes]

Yes. It can also mean behind the scenes in a non-literal sense.

For example:

• Las negociaciones se hicieron entre bastidores.
  = The negotiations were done behind the scenes.

But in El atrezo está entre bastidores, the meaning is clearly literal, because atrezo belongs to the theatre/stage world.