It looks like Subscribe's `to` prop expects an array. So, `[CounterContainer]` is just an array with a CounterContainer in it.

The `{ }` that wrap it are just part of the syntax of passing props. If you wanted to pass an object literal to a prop, you'd have to double-up the curly braces:

  <div style={{ color: 'white' }} />

And strings can be passed with or without curly braces:

  // both of these are fine:
  <div id='foo' />
  <div id={'foo'} />

They are used to create arrays. You are able to subscribe to multiple containers:

  <Subscribe to={[CounterContainer, SomeOtherContainer]}>
      {(counter, someOther) => ()}
  </Subscribe>

I believe that the [] indicate an array.

Ah, right! It just evaluates to JS, fair enough. Thank you all!

Almost. It evaluates JS expressions (not statements). It's a minor distinction but trips a lot of people up.