The large white space directly under the Title, is used to enter the body text for the post. It is important to know that this textbox has two ways to input and format/style text. If you notice at the top right, there are two tabs: Visual and Text. The Visual tab is used for styled text (RTF-Rich Text Format). That means that the text can be styled using bold, italics and underlined. Text can be different colors, sizes and font styles.
The Text tab is used mainly for codes, but can also be used to input/paste Plain Text. It is important to know that the Post textbox is HTML-enabled. That means, that you can use links and HTML codes. Note: when using HTML codes, always use the Text tab.