A World boss is a large angry boss, usually specific to an area or World. They are usually something that takes anywhere from 8 to 20 people to take down. Much like a Raid boss they have a lot of health on their side. The difference is World Bosses can usually be found during open world times, like while on patrol. They are usually viewed as a quick and entertaining challenge, Check out youtube for some ideas on what world bosses are, or Google World boss MMO. This should help you in understanding them a little better. Thank you for your question! You are the first person that has actually bothered to ask for information on World bosses!

Sounds cool actually, so kinda like a buff on the regular "eliminate the target before he runs to the teleporter" thing.