If you have multiple accounts, a unified inbox would show all accounts combined into a single view. We don't support this feature, but it is on our development roadmap. Email us if you want this feature.
As a workaround, you could create your own unified inbox by, for example, forwarding your accounts to a new gmail account that you create. You can even use gmail to check other IMAP and POP accounts directly, just look under Gmail Settings > Accounts > Check email from other accounts.