You've already seen the pronoun si used in verbi riflessivi  (reflexive verbs). However, si is not strictly a reflexive pronoun; it also has other functions. One of which is to take the place of a specific subject to give a sentence a more general meaning. This application is used to express: people in general, everyone, etc…; the English equivalent is “one”, "people". or the colloquial “you”.

Esempi:
                                                                                                In questo ristorante si mangia bene.                       
       
                                                                                       
 Non si può fumare qui.   
​                                                
In Italia, generalmente si va in vacanza in agosto.       

In this restaurant one eats well.
​In this restaurant people eat well.
​In this restaurant “you” eat well.

One cannot smoke here. / You can't smoke here.

In Italy, people generally go on vacation 
​in August

The si impersonale is formed by pairing si with any 3rd person singular verb form – that is, the lui/lei/Lei verb form:

si mangia                           
si parla                             
si capisce                          
si va                                  
si può                                

one eats/people eat
​one speaks/people speak
​one understands/people understand
​one goes/people go
​one can/people can/it is possible (to)

Pratica!