Here I mentioned "greater than" (or "bigger than") selector, but while I was reading SMACSS I realized that I don't understand some selectors good enough.

So, here are my examples taken from SMACSS.

Same HTML I am going to use, but with different CSS:

<!DOCTYPE html>
<html lang="en">
<head>
    <link rel="stylesheet" type="text/css" href="/css/index.css">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
    <div id="sidebar">
        I am a sidebar div
        <div>
            and I am inner div
            <div>
                and I am third level div
            </div>
        </div>
    </div>
</body>
</html>

1. With "greater than" selector:

CSS:

#sidebar>div {
    border: 1px solid #333;
}

Looks like this:

I am a sidebar div
and I am inner div
and I am third level div

2. With space

CSS:

#sidebar div {
  border: 1px solid #333;
}
Looks like this:
I am a sidebar div
and I am inner div
and I am third level div