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

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Non si può fumare qui.   
One cannot smoke here. / You can't smoke here.

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In Italia, generalmente si va in vacanza in agosto.   
​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 
​one eats/people eat                    

si capisce 
one understands/people understand ​

si va  
​one goes/people go

si parla 
one speaks/people speak 

si può  
​​one can/people can/it is possible (to)              ​

Pratica!