Can anyone explain this TypeScript statement?

Let's break this down bit by bit:

function MappableMixin(Base: T)

The function takes a generic type T a its argument

function MappableMixin<T extends Something>(Base: T)

The <TypeArgument> term just between the function name and the (params) term tells typescript to expect that the return type of this function of of type TypeArgument. In the case that TypeArgument is of the form T extends Something, it means that the returned value will be a type that extends Something, meaning it has all the properties of Something. Essentially in this case, MappableMixin takes in some argument, and returns that argument with the extra properties of the type Something.

<Model<MappableTraits>>

This is a generic type with a type argument. So for example, you may have a type Model, which accepts a type as an argument:

interface Model<T> {
  thing1: string;
  thing2: T
}

In the case of using <Model<MappableTraits>>, Model<T> would have the type

interface Model<MappableTraits> {
  thing1: string;
  thing2: MappableTraits
}

So Constructor<Model<MappableTraits>>> follows this same pattern, just 2 layers deep.

So the whole phrase means that MappableMixin is a function that returns an extended version of the value you gave it. The extension is specifically of the type Constructor<Model<MappableTraits>>>, which is a 2-layer typescript generic.