When two people or things are neck and neck, they are so close or similar that it's impossible to tell who's better or winning. A neck and neck race is too close to call.
In a neck and neck horse race, the horse's necks are right next to each other, because no one has been able to pull ahead. This phrase applies to other types of competition that are also tight. If two baseball teams are tied, they're in a neck and neck race for best in the division. Two applicants for a job could be neck and neck if their qualifications are excellent and similar. A neck and neck election can't be predicted.