La vendeuse de la poissonnerie dit que le saumon de ce matin est plus frais.

Vendeuse is the feminine form of vendeur (salesperson / seller). French nouns for people often change form depending on gender.

• le vendeur = the male salesperson
• la vendeuse = the female salesperson

So this sentence tells you that the fish shop worker is a woman.

Une poissonnerie is a fish shop or fishmonger’s shop. It refers to the store where fish and seafood are sold.

So:

• la vendeuse de la poissonnerie = the saleswoman from the fish shop / the fish shop assistant

French often uses longer noun phrases where English might prefer a single word.

• la vendeuse de la poissonnerie literally means the saleswoman of the fish shop
• In natural English, you might translate it as the woman at the fish shop, the fish shop assistant, or sometimes simply the fishmonger, depending on context

French is just being more specific about her role as the person selling the fish.

Here de la means of the or from the.

• de = of / from
• la = the (feminine singular)

Because poissonnerie is a feminine singular noun, you get:

• de la poissonnerie

This is not the special partitive de la meaning some. Here it is simply de + la.

French usually uses que after verbs like dire when introducing a full clause.

• dit que... = says that...

So:

• La vendeuse ... dit que le saumon... est plus frais
• The saleswoman ... says that the salmon ... is fresher

Just like in English, that can sometimes be omitted in translation, but in French que is normally required here.

French uses articles much more often than English does. With a noun like saumon, French normally needs an article:

• le saumon = the salmon

Even when English might say simply salmon, French often still uses le, la, un, du, etc., depending on the meaning.

Here le saumon probably refers to the salmon being sold / the salmon in question.

De ce matin means from this morning or from this morning’s catch/delivery.

• ce matin = this morning
• de ce matin = from this morning

In this sentence, it describes the salmon:

• le saumon de ce matin = the salmon from this morning

This suggests it arrived or was prepared earlier that same day.

The demonstrative adjective is:

• ce before most masculine singular nouns
• cet before masculine singular nouns starting with a vowel sound
• cette before feminine singular nouns

Since matin starts with a consonant sound, French uses:

• ce matin

Compare:

• ce livre = this book
• cet été = this summer

Because the subject of the clause is le saumon, which is singular.

• le saumon ... est plus frais = the salmon ... is fresher

Even though the sentence starts with la vendeuse, that is the subject of dit. Inside the second clause, le saumon is the subject of est.

So the structure is:

• La vendeuse ... dit = the saleswoman says
• que le saumon ... est plus frais = that the salmon is fresher

The adjective must agree with the noun it describes.

Here, frais describes le saumon, which is masculine singular, so the form is:

• frais = masculine singular

Other forms include:

• fraîche = feminine singular
• frais = masculine plural
• fraîches = feminine plural

Examples:

• Le poisson est frais.
• La soupe est fraîche.

It means fresher.

French often forms comparisons with:

• plus + adjective = more + adjective

So:

• plus frais = fresher / more fresh

In natural English, fresher is the better translation.

Yes. A comparative like plus frais usually implies that it is fresher than something else, even if the second part is not stated.

For example, the unstated comparison could be:

• fresher than yesterday’s salmon
• fresher than another fish
• fresher than the salmon at another shop

French, like English, can leave the second half of the comparison understood from context.

Yes. In many cases, the French present tense can translate to either the simple present or the present progressive in English, depending on context.

So dit can mean:

• says
• is saying

In this sentence, says is the most natural translation.

This is a very normal French sentence order:

• subject: le saumon de ce matin
• verb: est
• complement/adjective phrase: plus frais

So it follows the same basic pattern as English:

• The salmon from this morning is fresher.

French word order is often quite close to English in simple statements like this.

Grammatically, yes, but in normal context it most naturally means the saleswoman at the fish shop.

French de can show different relationships, including of, from, or associated with. Here the practical meaning is that she works in that shop.

So the best understanding is:

• the woman who sells fish at the fish shop

A few natural translations would be:

• The woman at the fish shop says the salmon from this morning is fresher.
• The fish shop assistant says this morning’s salmon is fresher.
• The saleswoman at the fishmonger’s says the salmon from this morning is fresher.

Since you said the meaning is already known, the main thing for the learner is noticing how French builds that meaning piece by piece.