两者都是返回 iterator 的一个元素，不过

yield 的返回值是当作一个元素
yield* 的返回值是一个 iterator，会依次返回这个 iterator 中的每个元素

举例

function* sub() {
    for (let i = 65; i < 70; i++) {
        yield String.fromCharCode(i);
    }
}

function* main() {
    yield "begin";
    yield sub();    // 返回的是 sub() 的结果，一个对象
    yield "---------";
    yield* sub();   // 依次返回 sub() 结果的的每一项
    yield "end";
}

for (var v of main()) {
    console.log(v);
}

The yield keyword is used to pause and resume a generator function.
The yield* expression is used to delegate to another generator or iterable object.

function* g1() {
  yield 2
  yield 3
}

function* g2() {
  yield 1
  yield g1()
  yield* g1()
  yield [4, 5]
  yield* [6, 7]
}

const iterator = g2()

console.log(iterator.next()) // { value: 1, done: false }
console.log(iterator.next()) // { value: {}, done: false }
console.log(iterator.next()) // { value: 2, done: false }
console.log(iterator.next()) // { value: 3, done: false }
console.log(iterator.next()) // { value: [4, 5], done: false }
console.log(iterator.next()) // { value: 6, done: false }
console.log(iterator.next()) // { value: 7, done: false }
console.log(iterator.next()) // { value: undefined, done: true }