World Tension is a numerical value that goes from 0 to 100, and it is in essence a measure of the world's fear of war breaking out.
World tension works mainly as a limiting factor on Democratic nations (they can only guarantee other nations at certain levels of tension, can only pass certain laws with high enough tension, and may or may not be able to join factions if tension isn't high enough). Actions mainly on the part of Fascist and Communist nations such as declaring war, doing certain focuses, and justifying war goals raises world tension. High tension is bad if you're Fascist or Communist, but good if you're Democratic.
The core of World Tension and EU4's aggressive expansion are the same, yes but there are many differences and intricacies.