To be "owned" is to be beaten very quickly or very easily. Someone may be owned by a variety of circumstances, for example running into a swarm of enemies, running into an area alone, or attacking an enemy far stronger than them. Common things to get owned by are Mursaat when your armor is not infused, Hydras casting Meteor, or a much better coordinated team in PvP.
The idiom mostly comes up in the phrase "You got owned" or as a stand-alone exclamation. It has been an Internet slang term and meme for several years, particularly in the online gaming community. Numerous variations, such as "ownz0red", "pwned", or "pwnt", are common. It is also popular to generate longer and more absurd variations, such as "al capowned", for humorous effect.
See also "Pwn" on Wikipedia.