A Keytip is a small popup near a component that indicates a key sequence that will trigger that component. These are not to be confused with keyboard shortcuts; they are instead key sequences to traverse through levels of UI components. Technically, a Keytip is a wrapper around a Callout where the target element is discovered through a 'data-ktp-target' attribute on that element.
To enable Keytips on your page, a developer will add the KeytipLayer component somewhere in their document. It can be added anywhere in your document, but must only be added once. Use the registerKeytip utility helper to add a Keytip. A user will enter and exit keytip mode with a IKeytipTransitionSequence, which is a key with any amount of modifiers (Alt, Shift, etc).
By default, the entry and exit sequence is 'Alt-Windows' (Meta) on Windows and 'Option-Control' on macOS. There is also a sequence to 'return' up a level of keytips while traversing. This is by default 'Esc'.
Fluent UI React components that have keytips enabled have an optional 'keytipProps' prop which handles registering, unregistering, and rendering of the keytip. The keySequences of the Keytip should be the full sequence to get to that keytip. There is a 'buildKeytipConfigMap' helper which will build a map of ID -> IKeytipProps to assist in defining your keytips.
For more details and examples visit the official docs. The R package cannot handle each and every case, so for advanced use cases you need to work using the original docs to achieve the desired result.