TinyMCE is a so called WYSIWYG editor for editing pages. WYSIWYG stands for What You See Is What You Get. It works similar to a word processor, where you can select the style for the content and see how it is going to look on the page.
Among available WYSIWYG editors CMS Made Simple has decided to use this TinyMCE, the editor that the TinyMCE module is using. TinyMCE is among the most developed WYSIWYG editors, with regular updates, a large following and customizable features.
However, it is very difficult to create a cross-browser online editor that works in all different kinds of environments. If you are the least familiar with HTML you can select no WYSIWYG in My Preferences/User Preferences in the Admin Panel. That gives you more control over the code that will be on the page.
There are also modules for other WYSIWYG editors available for download.