The Umbraco Rich Text Editor is the control you will use to add most of your content onto your website. I think most people will have used Microsoft word, or, Google docs so the interface should be pretty straightforward for you to work out. In the rich text editor, you can write, edit and format content however you want.
The Episerver rich text editor may look familiar to some people Instead of writing their own rich-text editor, Episerver uses a third-party product called TinyMCE which is used throughout the internet. Umbraco, Sitecore and WordPress all use the same editor, so there is a lot of information out there. The editor is pretty simple to use, but, if you ever get stuck I would recommend including the term ‘TinyMCE’ when you Google your problem, as you are a lot more likely to find a solution
Your rich text editor might have a few extra or less button depending on how your IT department has configured your website, but, by default your editor should look pretty similar to the screenshot above., for example, some companies may allow you to create tables within the editor and leave the table buttons as active, while other companies disable it.