Content that ends up in the trash does not get immediately deleted. The content is hidden from view for visitors on your site, but it is still preserved within the “Trash” tab when you’re viewing “All Posts/Pages”, ready to be restored and made visible again if you decide to bring the content back. Trash is one of the post statuses.
To restore a piece of content that you have moved to the trash, click on the content type (posts, pages, media, and so on) in the main menu when you’re viewing the administration panel. Next, click the “Trash” link with a number within a parenthesis next to it, close to the top of the screen. Locate your content in the list, hover your mouse pointer on it, and click “Restore”.
If you instead would have clicked the “Delete Permanently” link, the post would’ve gotten completely deleted from your database. If you delete a post permanently and later decide that you want to restore it, you’re only hope is that you either have database backups or a copy of the post contents saved somewhere else. Only delete a post permanently if you’re completely sure that you’re not going to want it back later.