Let’s say that I have a gmail address: itzbiswajeet@gmail.com.

Now if I am signing up for a newsletter at ABC Corp. I can register with the email address itzbiswajeet+abc@gmail.com. You can put any extra alphanumeric character after the plus sign and it still gets delivered to itzbiswajeet@gmail.com.

One benefit of this feature is to track where emails are coming from.