The regex defines what can and can't appear in the email address; this part:
...defines what characters can appear before the "@". The "@" ensures that there has to be an instance of the "@" symbol.
This part defines what can appear after the "@":
Note that neither of these parts allow "@"
This isn't the only regex that can validate emails. If you google "regex" and "email" you will find a whole bunch of other examples, some more thorough than others.