Learn Hardcore Italian: Il fruttivendolo pesa le mele sulla bilancia e mi dà il sacchetto. - The greengrocer weighs the apples on the scale and gives me the bag.

In Il fruttivendolo, it tells you we are talking about the greengrocer / the fruit seller, not just any one in general. Italian uses articles very regularly, often more than English does.

What does fruttivendolo mean exactly?

Fruttivendolo literally refers to a person who sells fruit, but in everyday Italian it often means a greengrocer or fruit-and-vegetable seller.

It is a masculine singular noun:

il fruttivendolo = the male seller / the greengrocer
la fruttivendola = the female seller
i fruttivendoli = the plural

Why is it pesa and not pesare?

Pesare is the infinitive, meaning to weigh.

In the sentence, the verb is conjugated:

io peso = I weigh
tu pesi = you weigh
lui/lei pesa = he/she weighs

So Il fruttivendolo pesa means The greengrocer weighs.