Dogs can safely eat celery in small amounts as long as you chop the celery up into bite sized pieces so that the dog can chew the celery much easier.
A dog could chew the celery themselves but it's much easier for the dog to eat the celery when you chop it up into bite sized pieces.
The celery is good for the dogs health in moderation although if the dog eats too much celery it could cause excessive urination in the dog.
The leaves should also be removed from the celery because the dog cannot easily digest the celery leaves and cutting the celery up helps the dog digest the celery easier as well.