One of the advantages to using the Content Editing Mode is that you can specify what content on the page can and cannot be edited.
This can be very useful for "locking down" certain parts of a page design, and moving them out of view in the content drawer, allowing you and your teammates to focus only on editing the relevant content.
For example, you might want to lock the logo image used at the top of a page so that it can't be changed, while allowing the rest of the images on the page to be editable.
Content that is locked can always be unlocked to make it editable again. And when you switch over to Studio Mode, you'll always be able to edit all of the content on a page, regardless of whether it's locked or not.
While in Content Editor Mode, you notice that every item in the content drawer has a "Lock" button at the right side:
To lock a piece of content, click this button. You'll be asked to confirm that you want to lock it:
Once you confirm the action, this piece of content will automatically be shuffled down to the bottom of the content drawer, and collapsed so that it's out of the way. To find it again, you can manually scroll down to the bottom of the drawer, or click the item on the page preview to automatically scroll to it.
You'll notice that the item is now greyed-out, and has a "lock" icon on it:
This piece of content can no longer be edited in Content Mode. (It can still be edited as normal if you switch over to Studio Mode.)
To make a locked piece of content editable once again, simply click the "unlock" button on the right-hand side.
You'll receive a similar confirmation message:
If you confirm the action, the item will return to its normal position in the content drawer, and you can edit it as usual.