If your mailbox is
firstname.lastname@example.org, Postfix will deliver
email@example.com, etc to this mailbox with the part after the
+ as the extension.
As far as the MTA is concerned, the extension is simply ignored. You can basically invent new unique mail addresses on the fly by creating new extension parts on a whim whenever you need to share your email address.
In Postfix, the extension separator is configurable. Out of the box, it is unset, though Debian ships a canned Postfix configuration where it is set to a plus character, which is also the character used e.g. by Sendmail; Qmail uses a dash.
Eli the Bearded used to publish an email addressing FAQ about this; it's long abandoned, and obsolescent in some parts, but you can still find it archived on faqs.org.