Outbound links are links from your website to another webpage outside of your domain.
It's a good rule to make sure all your outbound links open in a new tab. That way, users won't be forced off of your site completely and can navigate their way back to the page they were on with ease.
Linking to another webpage counts as a 'vote' for that content and boosts the ranking ability of that page. If you don't want your link to be counted as a vote, or don't want the search engines to associate your site with the page you are linking to you should use a nofollow tag.