This probably refers to a concept known as "audience extension" whereby a publisher retargets users who have visited their website on other websites that offer inventory for sale via an ad exchange. read more
That may refer to ads, that are displayed to website users while they are not using the website. For example you have a facebook account, but now you’re reading an article on some other website. Facebook can show you ads on that website and that will be the offsite targeted ad. read more