Email tags are a handy way to create a new account identity, while still managing all of your messages under one email address.
If you have the email address someone@example.com, you can create a new Google account with the address someone+sometag@example.com by entering the address with the tag here. This new account will have its own set of login credentials and will function as a totally new Google account.
The advantage of using email tags is that this doesn't create a new email address that you have to monitor. Everything sent to the email address with the tag is delivered to your regular email address, so you still only have a single email address to check for messages.
Please note that your email provider must support email tags for this to work. This will work for any @gmail.com address. To check that your email provider supports email tags, simply check that an email sent to someone+sometag@example.com is delivered to your email someone@example.com.