reacting to an argument..
Above each argument(see below), you will see the pseudonym of the arguer who made it, followed by a [Ω react] link. If you click on that, it will take you to that argument's permalink, where you can react by posting comments or follow-up arguments.
an argument opposing a proposition an argument supporting a proposition
Once you click react, you will see a something like below (again seen here in an iframe). Go ahead and read the argument, and react by commenting and following up, to get a clue as to what Argumentum is all about.