Your account access key is a unique, 32-character key that gives you the right to send data into your account. It does not give you rights to read data from your account (your username/password authentication allows data to be read from your account). Your account access key should remain private to prevent unwanted data from being streamed into your account. In the event that your access key is compromised, you can remove a key and create other keys in your account page (https://app.initialstate.com/#/account). You can view all active account keys in your account page.
Your account access key can be used to send data to an unlimited number of data buckets. Any device or service can start, stop, reboot, and continue appending to the same data bucket by always using the same bucket key and access key when sending data.
Related: Finding an access key