What is GMail Plus addressing? Say you have an email address like email@example.com. If you append a "plus" sign to your email username, gmail will ignore anything written between the + and @ sign.
So any email address sent to firstname.lastname@example.org or email@example.com or firstname.lastname@example.org will still reach your email@example.com inbox though technically, they are three different email aliases.
When you share your email with some non familiar service, like a newsletter, you can supply your existing email with a plus sign. If you ever receive spam addressed to that email alias, you know the exact source that's sending the spam and can easily block all emails using a GMail filter.
[type the alias in the To: fiedd and redirect all incoming message to Trash or apply a new label]
More GMail Easter Eggs [including the dot trick].