Activate
Activation confirms the email address on your new account, which is what opens the rest of the site after you register.
A new account is not fully open until the email address on it has been confirmed, because that address is how you recover a forgotten password and how the account is shown to belong to a real person. Registration sends you a message with a single-use link; opening it confirms the address and unlocks the signed-in pages. If the message did not arrive, you can request another from this page.