Simply means every link you add to your site that is not of your own domain name. In other words when clicked it will send users to some other site not yours. This good for the site on the recipient end not so good for yours.
Outbound links are the links on your website which are related to other website. These links are informative and should be related to a authority website.
Outbound links are links that are meant to take you elsewhere. These are links that are going to direct you to another specific webpage or website altogether. Most, if not all, websites have outbound links. Two sides of a coin.