From Craft CMS documentation page about requesting controller action:

when you send action as parameter format should be like:

 <input type="hidden" name="action" value="plugin-handle/controller/action">

and you can directly open that URL - with 'actions' at the beginning as default actionTrigger - directly because:

By default, Craft makes an actions/ route available for appending any valid action path.

so http://craftsite/actions/plugin-handle/controller/action can be resolved when open that URL in a browser.

From Craft CMS documentation page about requesting controller action:

when you send action as parameter, format should be without '/actions':

 <input type="hidden" name="action" value="plugin-handle/controller/action">

and you can directly open that URL directly- with 'actions' at the beginning as default actionTrigger - because:

By default, Craft makes an actions/ route available for appending any valid action path.

so http://craftsite/actions/plugin-handle/controller/action can be resolved when open that URL in a browser.