Hi Rahul,
While a prep book can be useful, my recommendation - when in doubt - is to follow the PMBOK Guide.
Anyway, the glossary at the end of the PMBOK Guide defines the fallback plan as an alternative set of actions and tasks available in the event that the primary plan needs to be abandoned (because of issues, risks, or other causes). See also Section 11.5.3.3, besides the references Lu suggested.
Hope this helps,
Gabriella