That's property spread notation. It was added in
ES2018 (spread for arrays/iterables was earlier, ES2015), but it's been supported in React projects for a long time via transpilation (as "JSX spread attributes" even though you could do it elsewhere, too, not just attributes).
{...this.props} spreads out the "own" enumerable properties in
props as discrete properties on the Modal element you're creating. For instance, if
this.props contained
a: 1 and b: 2, then
<Modal {...this.props} title='Modal heading' animation={false}>
would be the same as
<Modal a={this.props.a} b={this.props.b} title='Modal heading' animation={false}>
But it's dynamic, so whatever "own" properties are in
props are included. Since
children is an "own" property in props, spread will include it. So if the component where this appears had child elements, they'll be passed on to
Modal. Putting child elements between the opening tag and closing tags is just syntactic sugar — the good kind — for putting a children property in the opening tag.
Spread notation is handy not only for that use case, but for creating a new object with most (or all) of the properties of an existing object — which comes up a lot when you're updating state, since you can't modify state directly:
this.setState(prevState => {
return {foo: {...prevState.foo, a: "updated"}};
});