The secondary flight plan/route in the FMS is exactly what ETOPS240 has quoted.
In summary, two flight plans can be loaded in to the FMS. One is active, the other isn't. The inactive route can contain anything - a diversion, an escape route, a mirror image of the active route, a modified version of the active route, or something completely irrelevant. To activate it, it's just short keystroke sequence.
We use the secondary flight mode all the time for forward planning on multi sector days or where a quick turn around is required on the ground, once loaded it takes a couple of key strokes to activate.
@Jug
The secondary flight plan is displayed on the MFD as "secondary" when activated you cannot pretend to use the primary flight plan while actually using the secondary.