What does the pipe(|) mean in typescript?

Question

This is called union type in typescript.

A union type describes a value that can be one of several types.

Pipe (|) is used to separate each type, so for example number | string | boolean is the type of a value that can be a number, a string, or a boolean.

let something: number | string | boolean;

something = 1; // ok
something = '1'; // ok
something = true; // ok
something = {}; // Error: Type '{}' is not assignable to type 'string | number | boolean'

Playground

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And here's an example similar to one in the question:

class Test1 {
    public a: string
}

class Test2 {
    public b: string
}

class Test3 {
}

let x: (typeof Test1 | typeof Test2)[];

x = [Test1]; //ok
x = [Test1, Test2]; //ok
x = [Test3]; //compilation error

x is an array containing constructors of Test1 or Test2.

The pipe represents 'or'. So in this context it says that either of the declared types is allowed. Perhaps it is easy to understand a union with primitive types:

let x: (string | number);

x = 1; //ok
x = 'myString'; //ok
x = true; //compilation error for a boolean