nav ul {
    list-style: none;
    display: flex;
    padding: 0;
}

What is this CSS commonly used for?

.navbar a {
    display: block;
    padding: 14px 20px;
    text-decoration: none;
}

Why use display: block on navigation links?

.navbar a:hover {
    background-color: #555;
}

What does this accomplish?

.navbar .active {
    background-color: #04AA6D;
}

What is the purpose of this CSS?

@media screen and (max-width: 600px) {
    .navbar { flex-direction: column; }
}

What does this media query do?