yes, I know, it’s a crowded field

[Ostap]

The worst line I ever wrote as a pundit — yes, I know, it’s a crowded field — was the first line I ever wrote about the man who would become the 45th president: “If by now you don’t find Donald Trump appalling, you’re appalling.”

The New York Times

Hello, teachers. How do you understand the boldfaced part? What is "it"?

 

[Tarheel]

It means there are a lot of people doing it.

 

[Skrej]

He's stating that he's written a lot of bad lines as a pundit, so the fact that this one is the worst makes it even more remarkable for how bad it truly is.

When there's a lot of competition for something, we can say it's a crowded field. He has written many bad lines, so the competition to be declared his 'worst' is intense.