The Canvas module is simply an empty post-like-editor where you can throw anything you like. Text, images, embeds, HTML code. Think of it as the ultimate free module you can later on position anywhere you want on your site.
Hint: If you combine a Canvas module with the SiteOrigin’s Page Builder plugin (downloadable for free from the WordPress Plugin Repository) you can go crazy with the possibilities!
Please note: the modules availability and their specific settings depend on each theme, not all themes include the same modules with the same settings.