Typescript & operator

This looks like it's from the Intersection Types portion of the Language Specification. Specifically, the & is an intersection type literal. As for what it does:

Intersection types represent values that simultaneously have multiple types. A value of an intersection type A & B is a value that is both of type A and type B. Intersection types are written using intersection type literals (section 3.8.7).

The spec goes on to offer a helpful snippet to better understand the behavior:

interface A { a: number }  
interface B { b: number }

var ab: A & B = { a: 1, b: 1 };  
var a: A = ab;  // A & B assignable to A  
var b: B = ab;  // A & B assignable to B

Because ab is both of type A and of type B, we can assign it to a and/or b. If ab were only of type B, we could only assign it to b.

The code you shared may be from this comment on GitHub, which mentions Intersection Types.