The Code Block allows you to add example code to your content. The code you add to this block wont be executed, it's purely for displaying lines of code within your content.
Once you've added your text to your block, there are further options available in the Settings Sidebar. You can change the Text and Background colours, and various Typography settings like Font size, Appearance, Line height, Letter case & Letter spacing. You can also change the spacing around the block by adding Padding within the Dimensions settings.