What does a pending form status mean?

PENDING means that the form (or an individual field) is currently undergoing asynchronous validation, a check whose result has not yet been received.

Example: If an AsyncValidator is set for a field, for example a check of whether a username is unique via a server request:

When the user enters a value, Angular runs the asynchronous validator, and until the request completes, username.status === 'PENDING'.

That is, PENDING is an intermediate state, showing that the form is temporarily "waiting for a response", and the final validity (VALID / INVALID) has not yet been determined.