What does enclosing a class in angle brackets "<>" mean in TypeScript?

I am very new to TypeScript and I am loving it a lot, especially how easy it is to do OOP in Javascript. I am however stuck on trying to figure out the semantics when it comes to using angle brackets.

From their docs, I have seen several examples like

interface Counter {
  (start: number): string;
  interval: number;
  reset(): void;
}

function getCounter(): Counter {
  let counter = <Counter>function (start: number) { };
  counter.interval = 123;
  counter.reset = function () { };
  return counter;
}

and

interface Square extends Shape, PenStroke {
  sideLength: number;
}
  
let square = <Square>{};

I am having trouble understanding what this exactly means or the way to think of/understand it.

Could someone please explain it to me?

That's called Type Assertion or casting.

These are the same:

let square = <Square>{};
let square = {} as Square;

Example:

interface Props {
	x: number;
	y: number;
	name: string;
}

let a = {};
a.x = 3; // error: Property 'x' does not exist on type `{}`

So you can do:

let a = {} as Props;
a.x = 3;

Or:

let a = <Props> {};

Which will do the same

This is called Type Assertion.

You can read about it in Basarat's "TypeScript Deep Dive", or in the official TypeScript handbook.

You can also watch this YouTube video for a nice introduction.