An HTML block is usually used with external functions. For example:
Many times the offsite source provides the HTML code all you have to do is copy and paste it although you may write it yourself.
Click directly on the HTML block and use the Editing icons to open the HTML window and make changes.
You may also use the justification icons on the small black rectangle to move the block left or right allowing other text / images to wrap around it. Or delete it if you wish.