Yes, the title says it all. A 'red notice' is an 'international wanted poster' of sorts, aimed with encouraging any country that may be harboring the individual to extradite them for punishment. As you can see, Julian here got in big trouble for 'Sex Crimes in Sweeden' (alleged rape)... and now he's got Interpol after him.
At this point, he may as well give up. While normally it is doubtful the US would extradite a citizen, Julian here already got on the US governments bad side when he leaked of a quarter-million US State Department diplomatic cables that revealed the private views of US diplomats towards foreign leaders around the world.
The Obama administration is exploring possible criminal charges against Assange under the Espionage Act... meaning he's pretty much a wanted man wherever he goes, and it's just a matter of time before he's arrested for one thing or another.