For the haters, overflow is the arena outside a packed arena where prople who can't get in can watch on screen.

[Woodigo]

Beautifully! massive overflow crowd outside of my rally in New Hampshire chants "four more years!"&"USA!" while watching the speech. For the haters,"overflow" is the arena outside a packed arena where prople who can't get in can watch on screen.

I don't know what the second sentence mean. Why is the phrase 'for the haters' inserted?

 

[grassy]

That piece of information is directed to the haters. It is to let them know what "overflow" is.

 

[bennymix]

DT is making a point based on the idea that people who hate him, lie about the crowds he attracts. He's pointing out to them what the 'overflow' term means, i.e. packed stadium with thousands, AND more eager fans outside.