When you edit a blog post, page, discussion etc, you normally use Visual Mode, but you have the option of viewing the HTML Editor: the code that generates the page. If you are not used to it HTML can seem a bit complicated, but as you start to use it you will get your eye in.
There are some things you can do in HTML that you can't do in Visual Mode including:
Some HTML codes are not supported by the editor, if you switch back and forth between HTML Editor and Visual Mode then errors and unsupported codes will be stripped out, so you can check your code before publishing.
If you think you've messed up, then you have the option to cancel the edit and start again!