The syntax {{> name}} is for partials, i.e. to include another template:
Handlebars allows for template reuse through partials. Partials are
normal Handlebars templates that may be called directly by other
templates.
whereas {{{name}}} is for including data without escaping it. If you wanted the data to be escaped, you'd use {{name}}:
Handlebars HTML-escapes values returned by a {{expression}}. If you
don't want Handlebars to escape a value, use the "triple-stash", {{{.
So they are different things.
{{{body}}} refers to a body property in the current context provided by your application (alternatively it could refer to a helper function but that's not the case here).
If you had {{>body}} in the code, it would mean you have a template named body that you want handlebars to load.
Note that, if you use Handlebars proper, all partials need to be registered by calling Handlebars.registerPartial (if your project uses express-handlebars, it does it all for you so you won't find this in the code).
Handlebars's documentation can be found at handlebarsjs.com.