The formContext is a part of executionContext and it replaces deprecated Xrm.Page starting from Version 9.
Xrm.Page object was used to represent a form or an item on the form. With the latest version, the Xrm.Page object is deprecated, and you should use the getFormContext method of the passed in execution context object to return reference to the appropriate form or an item on the form.from Client API form context – Microsoft Docs
For your registered form events you need to check “Pass execution context as first parameter”.
You can get the fornContext with using getFormContext() function on executionContext parameter.