Your account_id is the globally unique ID of your account. You'll have to pass an account_id with most API calls.
Most merchants only have one account_id, which you can find on your dashboard. If you create more than one account (for an additional business, for example), you'll have more than one account_id.
Platforms and marketplaces that create accounts for users will receive a different account_id for each users. You'll receive a user's account_id in response to the /account/create call.
You can find all accounts created for a user by making the /account/find call with that user's access_token.