dns is a system for 'name resolution'. when you 'request' facebook.com by typing it into the url bar or clicking a link, dns servers look at the string of letters 'facebook.com' and determine what server to send you to based on a dns record that facebook provides. BGP is a routing system, which means it determines what different servers your request should go through to get to facebook quickly. "It's a BGP/DNS issue" means that for some reason facebook's DNS records are pointing to BGP servers that aren't online (either the record is wrong or the server is down)
(im not a network expert and also im leaving some stuff out so somebody feel free to correct me or explain further)
Oh okay that is understandable to me thank you very much