Remember Michael Flynn? If you don't, it's totally understandable, because he's kind of in hiding now. He was the retired United States Army Lieutenant who was chosen by Trump to be the National Security Advisor. Trump was a huge supporter of Flynn. Well, for a few days.
The Donald's opinion of Flynn most likely soured after Flynn resigned from office amidst controversy during the whole Trump-Russia investigation scandal. There could be many factors that lead to his departure, but most of them don't seem too good for Flynn - like that he may have lied to the Pentagon about his ties to Russia.
And because it seems like the Trump administration believes that a bunch of US reporters are more dangerous to the security of other countries than elected officials possibly colluding with other nations, specifically ones that have shown a disdain for America and were once involved a certain war of the cold variety not to long ago, it only makes sense that Michael Flynn's son, Michael Flynn Jr. would send out a tweet like this.