Stephen, you’re very welcome! You are correct, as well in that since you are editing a child theme, it will not be overwritten if you update a stock theme such as twenty-eleven.
As far as an explanation for the code, it was previously selecting all anchor (or ‘a’) elements contained within the list item with a class ‘current-menu-selector.’ So by placing the ‘>’ it selects only the anchor element immediately within the list item. So any anchor elements immediately within the list item will be selected but any anchor elements further down the nest (such as in another unordered list) will not be selected.
For example:
<ul>
<li class="current-menu-item">
<a href="#">This will be selected since it's an immediate ancestor</a>
<ul class="sub-menu">
<li>
<a href="#">This will not be selected since it's not an immediate ancestor. It's an ancestor of an additional unordered list.</a>
</li>
</ul>
</li>
</ul>
So essentially by just placing a space, it selects ALL following anchor elements held in that element. By placing the ‘>’ it selects just the ones closest to the parent element.