Here is a look at the red and green lines in the Timeline, plus the WAB (Work Area Bar) that can be set to limit the area of the Timeline, that is Rendered:
This is from PrPro 2.0
This is from PrElements 4.0.
The WAB will be almost identical in later versions of PrPro and PrE.
Hope that this helps,
Note: The yellow lines are now in PrE too, and replace the old "red line." This was done, when PrE 11 was introduced, and is powerful enough, and smart enough, to adjust a Project per the first Clip added, unless one checks "Force Project Settings," at New Project.
For the Mac, the keyboard shortcut is the Return key (same as Enter key on the PC).