The circles are called places and represent the inboxes in the office-metaphor. The rectangles are called transitions and represent the tasks to be performed. Places are inactive . All the places do is hold tokens representing the state of the process. If, for example, there's a token in place D above, then that means we're ready to pack the order.
Transitions are active .
When this happens, the transition is said to fire . Transitions can only when there's at least one token in each input place . When that is the case, the transition is enabled . That the transition is enabled means it is able to fire . The time the transition is enabled and the time it fires are different.