Basic Tenets of Waterfall model are as follows-
1. Requirements specification
5. Testing and debugging (AKA validation)
This is the central idea behind Big Design Up Front (BDUF) and the waterfall model - time spent early on making sure that requirements and design are absolutely correct will save you much time and effort later. Thus, the thinking of those who follow the waterfall process goes, one should make sure that each phase is 100% complete and absolutely correct before proceeding to the next phase of program creation.
Program requirements should be set in stone before design is started (otherwise work put into a design based on "incorrect" requirements is wasted); the program's design should be perfect before people begin work on implementing the design (otherwise they are implementing the "wrong" design and their work is wasted), etc.
Requirements defect that is left undetected until construction or maintenance will cost 50 to 200 times as much to fix as it would have cost to fix at requirements time.") To take an extreme example, if a program design turns out to be impossible to implement, it is easier to fix the design at the design stage than to realize months later, when program components are being integrated, that all the work done so far has to be scrapped because of a broken design.