it’s super useful for people who want their assistants to have access to read or respond to mail on their behalf. Now this functionality is available for anyone using Gmail. To grant access to another account, click the Settings link in the top right corner of Gmail. On the "Accounts" tab, you’ll see a new section where you can "Grant access to your account."
If you are using more than one Gmail accounts (applicable for Google apps too) you allow access to your inbox with one login.
To enable this
1. Visit the Settings of Gmail
2. Go to Accounts tab
3. Click "Add Account" under "Grant access to your account:"
4. Enter the email address and click "Next Step"
5. On the next screen click "Send email to grant access"
The account you add will get a verification email with links to accept or deny access. Once the account accepts, a small down arrow will appear beside the email address at the top right corner of Gmail which can be used to toggle between accounts.
Read more on Official Gmail Blog