Why is nitrous acid prepared in situ? Why is nitrous acid prepared in situ?

I am not sure about the above question. First of all, I don't exactly understand what is meant by &quot;in situ&quot;. Secondly, how would preparing nitrous acid in situ, in the diazotisation of amines with nitrous acid, make it less toxic or unstable?
Well, if you don't make it in situ, you would have to add the nitrous acid you have already kept in your laboratory shelf, and surprise! It's not there! Because, it has already decomposed. 

Nitrous acid ($\ce{HNO2}$) has an intermediate oxidation state of +3, and hence, it tends to disproportionate into more stable oxidation states:

$$\ce{2HNO2 -&gt; NO2 + NO + H2O}$$

$\ce{NO2}$ itself dissolves in water to give $\ce{HNO3}$ and $\ce{HNO2}$. This gives an overall reaction in aqueous solution as:

$$\ce{3HNO2 -&gt; HNO3 + 2NO + H2O}$$

This should give you a good idea on why we make nitrous acid within the reaction mixture. You can't store it beforehand. It must be made when it's time to use it.