How to understand JS realms

In ECMAScript specification there is notion of "realms" introduced:

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

In Rauschmayer's book "Speaking JavaScript" author writes about objects which can cross realms:

In web browsers, each frame and window has its own realm with separate global variables. That prevents instanceof from working for objects that cross realms.

What exactly constitutes "realm"? What else besides frame can separate websites code to another realm and what are the consequences?

The language reference uses abstract terms because JavaScript environments can vary widely. In the browser, a window (a frame, a window opened with window.open(), or just a plain browser tab) is a realm. A web worker is a different kind of realm than a window, but it's a realm. Same goes for service workers.

It's possible for an object to cross realm boundaries because windows opened from a common base window can intercommunicate via function calls and simple variable references. The mention of instanceof in that excerpt you quoted has to do with that. Consider this code in an <iframe> window:

window.parent.someFunction(["hello", "world"]);

Then imagine a function in the parent window:

function someFunction(arg) {
  if (arg instanceof Array) {
    // ... operate on the array
  }
}

That won't work. Why? Because the array constructed in the <iframe> window was constructed from the Array constructor in that realm, and therefore the array is not an instance constructed from the Array in the parent window.

There's a much stronger "wall" between web worker realms and window realms, and such effects don't happen in those interactions.