There are two ways you can utilize to write/edit your posts on WordPress, the Visual and the Text editor.
The visual editor is a WYSIWYG editor, which emulates a Word-like functionality, but if you want to get fancy with your text, for example, want to use specific HTML or CSS etc, you can’t do that. This is where the text editor comes in, the text editor is essentially a raw input box, where you can use HTML tags to format your content, CSS to apply styles, either via IDs/classes or even inline.
In many cases we need to switch from one to another, this is very simply done by clicking the tabs on the top right corner of the editor, as seen below.
more info on WordPress editors here.