The forward() function is made available for importing by specifying __all__ in the turtle module, relevant part from the source code:
_tg_turtle_functions = [..., 'forward', ...]
__all__ = (_tg_classes + _tg_screen_functions + _tg_turtle_functions +
_tg_utilities + _math_functions)
Currently, pycharm cannot see objects being listed in module's __all__ list and, therefore, marks them as an unresolved reference. There is an open issue in it's bugtracker:
Make function from method: update __all__ if existing for starred import usage
See also: Can someone explain __all__ in Python?
FYI, you can add the noinspection comment to tell Pycharm not to mark it as an unresolved reference:
from turtle import *
#noinspection PyUnresolvedReferences
forward(100)
done()
Or, disable the inspection for a specific scope.
And, of course, strictly speaking, you should follow PEP8 and avoid wildcard imports:
import turtle
turtle.forward(100)
turtle.done()