The account_id is the globally unique identification of your WePay merchant account. A WePay merchant account is a place to hold funds until withdrawal. It is also referred to as a payment account or sub account. Each user on WePay can open up as many WePay merchant accounts as they want and have them associated with various applications.
To see which WePay merchant accounts your app has opened for users, go to the "Data" section of your app dashboard. Using the top left dropdown menu, select "accounts"—you can view all accounts you have created from here.
You can also find all accounts opened for a user by making the /account/find call with the access_token associated with the user you want to find accounts for. This call will return an array of accounts, including any account_ids.
To open a merchant account for a user on your application, you can make the /account/create call.