Au secrétariat, une employée vérifie son dossier d’inscription avant la rentrée.

Because au is the contraction of à + le.

• le secrétariat = the administrative office / the secretariat
• à le secrétariat becomes au secrétariat

Here, au secrétariat means something like at the office or in the administrative office. French often uses à for location in this kind of context, even where English might prefer in.

In this sentence, le secrétariat does not mean secretariat in a political or international sense. It usually means the administrative office of a school, university, or organization.

So Au secrétariat suggests the scene is taking place at the school’s or institution’s admin office.

The comma separates an introductory phrase from the main clause.

• Au secrétariat sets the scene: At the administrative office
• Then the main action follows: une employée vérifie son dossier d’inscription

This is similar to English sentences like:

• At the front desk, an employee checks her registration file.

The comma is not always absolutely required in every short sentence, but it is very natural here.

Une employée means a female employee.

French often marks feminine forms in writing:

• un employé = a male employee
• une employée = a female employee

The extra e shows the feminine form. In pronunciation, employé and employée are usually pronounced the same.

Yes, son can mean either his or her.

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

Here:

• dossier is masculine singular
• so the possessive must be son

So:

• son dossier = his file or her file, depending on context

Because the subject is une employée, we understand it as her file here.

A dossier is a file, folder, or set of documents.
Inscription means registration or enrollment.

So un dossier d’inscription is a registration file, enrollment file, or application file, depending on context.

It usually refers to the paperwork connected with signing up for school or a program.

Because de becomes d’ before a vowel sound.

• de + inscription
• becomes d’inscription

This is called elision. French does this very often to avoid two vowel sounds next to each other.

Other common examples:

• l’école instead of le école
• j’aime instead of je aime

Vérifie is the present tense of vérifier, meaning to check or to verify.

So:

• une employée vérifie = an employee checks

The form vérifie is:

• third person singular
• used with elle, il, or a singular noun such as une employée

French often uses the present tense for:

• describing what is happening in a scene
• giving an example sentence
• stating a habitual or typical action

So une employée vérifie... can mean:

• an employee is checking...
• an employee checks...

English sometimes chooses between simple present and present continuous more clearly than French does.

Literally, it means before the return, but la rentrée has a special meaning in French.

It usually refers to:

• the start of the school year
• the return to school after summer
• more broadly, the time when normal work and public life begin again after the summer break

So avant la rentrée means before the start of term or before the school year begins.

French often uses the definite article where English does not.

Here, la rentrée refers to a known event: the start of the academic year. It is treated as a standard, familiar moment in the calendar.

So French naturally says:

• avant la rentrée

where English might simply say:

• before school starts
• before term begins

Yes. For example:

• Une employée vérifie son dossier d’inscription au secrétariat avant la rentrée.

That is also grammatical.

The original version starts with Au secrétariat to set the scene first. This gives a slightly more descriptive or narrative feel:

• At the administrative office, ...

So the fronted phrase adds emphasis to the location.

Not always. Avant can be followed by:

• a noun: avant la rentrée = before the start of term
• de
  • infinitive: avant de partir = before leaving

In this sentence, it is followed by a noun phrase: la rentrée.

So:

• avant la rentrée = before the new term
• avant de vérifier = before checking

Both patterns are common, but they work differently.