La rougeur disparaît quand elle met une serviette froide sur son front.

Rougeur is a noun, not an adjective. It means redness, flushing, or a red patch/irritation.

• rouge = red or red-colored
• rougeur = redness

So la rougeur is the redness, not the red.

French usually needs an article before a singular noun, where English sometimes does not.

So French says:

• La rougeur disparaît

literally, The redness disappears

Even if English might say just Redness disappears in some contexts, French normally prefers la rougeur.

Disparaît is the 3rd person singular present tense of disparaître.

That means it goes with:

• il disparaît = he disappears
• elle disparaît = she disappears
• la rougeur disparaît = the redness disappears

Here the subject is la rougeur, which is singular, so disparaît is singular too.

That accent is part of the standard spelling of the verb disparaître in traditional French spelling.

It does not change the basic meaning here. It is mainly a spelling feature connected to the history of the word.

For a learner, the main thing to remember is simply:

• infinitive: disparaître
• present: je disparais, tu disparais, il/elle disparaît

French often uses the present tense for something that happens generally, habitually, or whenever this condition happens.

So this sentence suggests something like:

• The redness disappears when she puts a cold towel on her forehead.

That can mean:

• this is what usually happens
• this is a general fact
• this is the normal result

This works very much like English.

Quand means when. It introduces a time clause.

So:

• quand elle met une serviette froide sur son front
• when she puts a cold towel on her forehead

It tells you at what time / under what circumstance the redness disappears.

Because met is the correct 3rd person singular form of mettre in the present tense.

Present tense of mettre:

• je mets
• tu mets
• il/elle met
• nous mettons
• vous mettez
• ils/elles mettent

So with elle, you must say elle met.

In French, many descriptive adjectives usually come after the noun.

So:

• une serviette froide = a cold towel

This is the normal order for an adjective like froide.

English usually says cold towel, but French usually says towel cold in that order.

Because froide agrees with serviette, and serviette is feminine singular.

Agreement here is:

• masculine singular: froid
• feminine singular: froide

Since it is:

• une serviette → feminine singular

the adjective must be:

• froide

Because front is a masculine noun in French.

French possessive adjectives agree with the thing possessed, not with the owner.

So:

• son front = her forehead or his forehead
• sa main = her hand or his hand

The gender of elle does not control the choice here. The gender of front does.

No. Son can mean his or her.

What matters is the gender of the noun after it:

• son front because front is masculine
• sa tête because tête is feminine

So son front here still means her forehead, because the person is elle.

In this sentence, yes, it clearly means some kind of towel, cloth, or cold compress.

Be aware, though, that serviette can also mean napkin in many contexts.

Context tells you the meaning:

• sur son front makes a dining napkin unlikely
• so here it means a cloth/towel placed on the forehead

Yes, French often uses the definite article with body parts, especially in constructions where possession is already obvious.

For example, French could say:

• Elle se met une serviette froide sur le front.

That is very natural.

In your sentence, sur son front is also correct and very clear. It explicitly says on her forehead.

So both patterns exist, but they are used in slightly different constructions.

Because this version is built with plain mettre:

• elle met une serviette froide sur son front
• she puts a cold towel on her forehead

French can also use a reflexive version:

• elle se met une serviette froide sur le front

That literally looks like she puts herself a cold towel on the forehead, but in natural English it still means she puts a cold towel on her forehead.

Both are possible; the sentence you were given simply uses the non-reflexive structure.

A rough English-style pronunciation is:

la roo-ZHUR dee-spa-RAY kahn el meh ewn ser-VYET frwad sur sohn frohn

A few helpful points:

• rougeur: the g sounds like the s in measure
• disparaît: the final -t is silent
• quand: the d is silent
• met: the final -t is silent
• front: the final -t is silent

So the sentence is smoother than it looks in writing.