What do they write in brackets {} in the render function in react?

Question

React

The documentation explains why they write code in the render function between { } rather superficially. It says that the { } code is written in curly braces so that JSX understands that this is JavaScript. Or they write that you can write a JS expression in brackets. But expressions in my understanding are something like this: a= b+c or a=b.
And I often see that in curly brackets they write just one word and that's it. Here are some examples:

render() {
 
    return (
     <WeatherDisplay
          key={activePlace}                      // ВОТ ЗДЕСЬ
          zip={PLACES[activePlace].zip}  //ИЛИ ЗДЕСЬ 
        />
      </div>
    );
}

render() {
    const { isActive } = this.state;              // И ЕЩЕ

    return (
      <div>
        <button onClick={handleClick}>Try it</button>    // И ВОТ
        <div className={isActive ? 'mystyle' : ''}>    
          This is a DIV element.
        </div>
      </div>
    );
  }
}

So what do they write between the curly braces?
What are your options for writing there?
Are these the result values ​​of the calculations in the function?
Are they object references?
Are these references to variables that we have manipulated and have now assigned a new value to some element?

Answer 1

in my understanding it is something like this: a= b+c or a=b.

An expression is any construct that evaluates to a value. One variable written just like that is also an expression:

const a = 1;
a; // это выражение

Actually, by default, react props accept strings, if you need to pass something else, then you need to use curly braces, even if it's a number.

Answer 2

But expressions in my understanding are...

Clear.
Forget about react for the next six months and go learn js.