What is a form control in HTML?

A form is a component of a Web page that has form controls, such as text fields, buttons, checkboxes, range controls, or color pickers. A user can interact with such a form, providing data that can then be sent to the server for further processing (e.g., returning the results of a search or calculation). No client-side scripting is needed in many cases, though an API is available so that scripts can augment the user experience or use forms for purposes other than submitting data to a server.

A form control is a user interface control that serves as the point of connection between the user and the server. Interactions vary by control type: buttons: button file handling: input type="file" menus: select, etc. And are also grouped into categories
Controls are essentially an API of key-value pairs for pinging back and forth to the server.

W3C's Form Section is incredibly informative in its walk through of forms, form elements, form controls, form owners, and more insight to the inner workings of the Internet's workhorse: the humble HTML form.

References:
Forms
HTMLFormElement
Association of Controls and Forms
XForms Glossary
Form Controls Infrastructure

For Bootstrap it seems to be a styling thing: From bootstrap's page:

Individual form controls automatically receive some global styling. All textual <input>, <textarea>, and <select> elements with .form-control are set to width: 100%; by default. Wrap labels and controls in .form-group for optimum spacing.

But more broadly it seems to be more, as per @the_velour_fog 's comment:

it seems to be more than just a styling thing: it seems form control just refers to the individual HTML elements in a HTML form e.g. from https://html.spec.whatwg.org/multipage/forms.html; A form is a component of a Web page that has form controls, such as text fields, buttons, checkboxes, range controls, or colour pickers.