What does "air traffic control delay" mean

@ucbalumnus he’s actually in Newark for two days befor he continues on to China so no worries there! He’s has a pre-departure orientation for the NSLI-Y program in Newark. I sent email to the program letting them know he’ll be arriving late. Since the problem is in Newark I’m sure there will be other kids arriving late too. If he had a connecting flight today I’d be pulling my hair out!