After you've finished putting in all of your transitions, you might want to see how the whole movie looks. You can preview your project by hitting the Enter key.
If you want to start somewhere in the middle, move the Edit Line to the point where you want to start and hit Play in the Project window of the Monitor.
To look at the whole project again, just hit Enter. Premiere won't have to render it again; it will just start playing at the beginning.