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. The thing that causes an enabled transition to fire is called trigger . This is called a user trigger and is symbolized with a fat arrow pointing to the task.