Period in postulate; what does it mean?
After an $\exists$ the period can be read as "such that".
For example, 
$\exists x\in \mathbb{N}. x&gt;4$ can be read as "there exists a natural $x$ such that $x$ is greater than 4."

After a $\forall$ it can be read as "it is the case that". For example,
$\forall x\in\mathbf{N}. x +  1&gt; x$ can be read as "for all natural numbers $x$, it is the case that $x+1$ is greater than $x$.

Generally one can omit "it is the case that" and the sentence still works.