This is an Early Access feature for existing Workflows users. To enable it, use the Early Access Feature Manager as described in Manage Early Access and Beta features.
The flows used to make a connector are built almost entirely of functions. At a high level, functions are used for manipulating and generating data that gets passed between the connector and service being integrated. The functions menu is split into several categories within the product, such as Branching, Error Handling, Text, Object, JSON, and Encryption.
For a full list of functions that are available in Connector Builder, see Functions in Workflows.