Lobo is an alien bounty hunter. He speaks Interlac. He refers to himself, and is known to other alien races such as the Kroloteans, as The Main Man.In fact, Lobo's full name in Interlac is Hahn Sho Lobo, which translates as The Main Man.
An unknown party hired Lobo to travel to Earth and capture or kill the Krolotean posing as Tseng. The contract specified that Lobo was to "put on a show for the locals", and he was happy to oblige, recklessly forcing his way through police and security forces. Despite interference from both Batgirl and Wonder Girl, Lobo successfully captured his quarry and returned to space.
In the comics, Lobo is a Czarnian bounty hunter, who can't be killed by anyone except other Czarnians. But since he's the last of his race, that won't happen; he killed everyone on his planet for fun. His name translates from a Khundish dialect as "He who devours your entrails and thoroughly enjoys it".
This is Lobo's third animated incarnation, and fourth overall appearance. Previous versions include Superman: The Animated Series and Justice League from the DCAU, and Legion of Super Heroes, where he had a small, unnamed appearance.