Tokens

If you're sufficiently brave to venture into the demonic depths of the UML specification, you'll find that the activity section of the specification talks a lot about tokens and their production and consumption. The initial node creates a token, which then passes to the next action, which executes and then passes the token to the next. At a fork, one token comes in, and the fork produces a token on each of its outward flows. Conversely, on a join, as each inbound token arrives, nothing happens until all the tokens appear at the join; then a token is produced on the outward flow.

You can visualize the tokens with coins or counters moving across the diagram. As you get to more complicated examples of activity diagrams, tokens often make it easier to visualize things.