> and ~ expressions in CSS

Your selector means:

Select any element
that is a child of an element with the class content
which follows a label
which in turn follows a :checked input element.

> is the child combinator. It selects elements that are children of a certain parent element. Unlike the space (the descendant combinator), it only selects immediately-nested elements. See this answer for an illustration on how it works.

~ is the general sibling combinator. It selects elements that follow after other elements within the same parent (i.e. are siblings). Unlike + (the adjacent sibling combinator), it doesn't require an element to immediately follow another in the same parent. Compare the illustration below to this other answer which covers the + combinator.

Be careful, as the ~ combinator does not just select any sibling element. It only selects an element that comes after its sibling, so :checked ~ label will not match a label that occurs before the checked input element.

An illustration:

<section>
    <input type="radio" name="example1" value="1" checked>
    <label>1</label>
    <input type="radio" name="example1" value="2">
    <label>2</label>
    <input type="radio" name="example1" value="3">
    <label>3</label>

    <div class="content">
        <h3>Subheading 1</h3>     <!-- [1] Selected -->
        <p>Some text              <!-- [1] Selected -->
           <em>with emphasis</em> <!-- [2] Not selected -->
        </p>
        <p>Some text</p>          <!-- [1] Selected -->
    </div>
</section>

<section>
    <input type="radio" name="example2" value="1">
    <label>1</label>
    <input type="radio" name="example2" value="2">
    <label>2</label>
    <input type="radio" name="example2" value="3">
    <label>3</label>

    <div class="content">
        <h3>Subheading 1</h3>     <!-- [3] Not selected -->
        <p>Some text              <!-- [3] Not selected -->
           <em>with emphasis</em> <!-- [2] Not selected -->
        </p>
        <p>Some text</p>          <!-- [3] Not selected -->
    </div>
</section>

What's selected and what's not:

1. Selected
   This h3 or p element is located directly inside a .content parent element. That .content element follows at least one label, and this label occurs after at least one input element that is :checked.

   Note that any of the radio buttons here can be checked, and the elements will match, because as mention above ~ doesn't require a label to follow it immediately. Also, given the structure, either one of the ~ selectors can be swapped out for a +:

   :checked + label ~ .content > *
   :checked ~ label + .content > *
   

   But this selector:

   :checked + label + .content > *
   

   Will only match if the third radio button is checked, because it's the only one that's immediately followed by a label and the .content element.

2. Not selected
   This em element is nested within one of the p elements which is itself contained within .content. Based on the illustration here, it won't be selected as it's not a child of .content.

3. Not selected
   Unlike in [1], none of the label elements in this section follow any :checked input element. Therefore, nothing is selected here, because it doesn't satisfy :checked ~ label.