Click it to confirm that you own your main account.
After confirming, you can use the Forwarding options on your other account to forward all email to your main email address.
If you add a Gmail address, all you’ll have to do is click the button.
You’ll get an email with a link you’ll need to click — it should appear in your main inbox now that you’ve set up forwarding.
You’ll be prompted to enter your main email address.