They’re very sour. Sour apples mixed with a few sweeter apple make cider. Those on the tree are eye watering sour, but there are others in the orchard that are sweeter.
Here is the man who owns the farm, he explains it very nicely. Apologies for using the thread for anything other than pics, but this is part of England, and though the video is 13 years old, the place is unchanged.