Le feu de croisement est allumé.

Literally, feu means light here, even though it often means fire in other contexts. In driving vocabulary, un feu can mean a lamp, signal, or vehicle light.

de croisement literally relates to crossing or passing. So feu de croisement is the light used when vehicles cross each other on the road without dazzling the other driver.

In everyday English, this is usually dipped headlight, low beam, or passing beam, depending on the variety of English.

French often uses the singular when talking about a type of equipment or a function in general.

So Le feu de croisement est allumé means something like:

• The dipped headlight is on
• or The low beam is on

Even though a car normally has two front lights, French can treat the system or function as a single thing.

You may also see the plural in other contexts, such as les feux de croisement, especially when talking about the lights as a pair or as a category.

Here, de links the noun feu to the type of light it is.

So:

• feu = light
• de croisement = for passing/crossing

This is a very common French structure:

• une salle de bain = bathroom
• une ceinture de sécurité = seat belt
• un feu de route = high beam

So de here is not really of in a natural English sense; it is just part of how French builds compound nouns.

Est allumé means is lit / is on. It describes the current state of the light.

• allumé = lit, switched on
• est allumé = is on

By contrast, allume without an accent is usually a verb form of allumer:

• il allume = he turns on / it turns on

So:

• Le feu de croisement est allumé = The low beam is on.
• Il allume le feu de croisement = He turns on the low beam.

It is the past participle of allumer, but in this sentence it functions like an adjective describing the state of the light.

So grammatically, you can think of it as:

• allumer = to switch on / light
• allumé = switched on / lit

With être, French often uses a past participle to describe a resulting state:

• La porte est fermée = The door is closed.
• La lumière est éteinte = The light is off.
• Le feu de croisement est allumé = The low beam is on.

Because feu is a masculine singular noun:

• le feu = masculine singular

So the adjective/past participle must agree with it:

• masculine singular: allumé
• feminine singular: allumée
• masculine plural: allumés
• feminine plural: allumées

If the noun were feminine, you would use allumée:

• La lampe est allumée.

A careful pronunciation is roughly:

luh fuh duh krwar-zuh-mahn ay ta-loo-may

A few important points:

• feu has the French eu sound, which does not exist exactly in English.
• croisement sounds roughly like krwah-zuh-mahn.
• The final -ent in croisement is not pronounced.
• allumé ends with -mé, pronounced may.
• There is often a liaison between est and allumé, so it sounds like ay ta-loo-may.

These are two different driving lights:

• feu de croisement = dipped headlights / low beam
• feu de route = full beam / high beam

The feux de croisement are used normally when other cars are present. The feux de route are stronger and used when the road is dark and clear, but they should be turned off when they would dazzle other drivers.

Yes. That is completely natural too.

The difference is mainly one of focus:

• Le feu de croisement est allumé: refers to the low-beam function/system in a general sense.
• Les feux de croisement sont allumés: emphasizes the lights as a pair.

Both can be understood, and both are used.

In road and vehicle vocabulary, French has several specific words:

• feu = light, signal, vehicle light
• phare = headlamp/headlight unit
• lumière = light in a more general sense

So:

• les phares often refers to the headlights themselves
• les feux de croisement refers specifically to the dipped/low-beam setting

A French speaker would normally use the technical driving term feux de croisement rather than lumières in this context.

It usually describes a condition or state, not the action of switching the light on.

So it means:

• the low beam is on
• the light has been turned on and is now on

If you want the action, French would use allumer:

• Il allume le feu de croisement. = He turns on the low beam.
• Le feu de croisement est allumé. = The low beam is on.