Legacy API Accounts in BigCommerce are how you take advantage of their Python and PHP APIs. Their different from their standard API Accounts in that you don't need to set up OAuth for your project. This is useful for things like batch scripts and not for User-Facing Tools.
To add a Legacy API Account to your BigCommerce store, all you need to do is log in to the backend, and go to
Advanced Settings > Legacy API Settings
Once there, all you need to do is pick a username and hit Save. Make sure to note down your API Token, since you'll need that to do anything with your store.
Make sure you keep your API Token secure. With it, anybody can add or delete Anything to your store. Treat it like you would your Bank Password. I've got several posts in the works of thing you can do with the BigCommerce Python and PHP APIs, so make sure to sign up for my newsletter or Follow Me On Twitter to stay up to date!