the Waterfall Model

is a breakdown of development activities into linear sequential phases, meaning that each phase is passed down onto each other, where each phase depends on the deliverables of the previous one and corresponds to a specialization of tasks.

the Model

is usually split into the six following phases

1. System and Software requirements
2. Analysis resulting in models, schema and business rules
3. Design resulting in the software architecture
4. Coding the development, proving and integration of the software
5. Testing the systematic discovery and debugging of defects
6. Operations, the installation, migration, support and maintenance of complete systems.

the Waterfall model states that one should move on to the next phase only when the preceding phase is reviewed and verified.

Sources

https://en.wikipedia.org/wiki/Waterfall_model