Waterfall Model.
The waterfall model is a linear and uncomplicated software development methodology.

The waterfall model organizes all phases sequentially, with each subsequent phase reliant on the results of the preceding phase.
It employs a top-down strategy.

 

The waterfall model offers a structured approach to project management, yielding a concrete outcome at the conclusion of each step.
Upon completion of the need, you will get a concrete output.

First you get all necessary requirements.
Subsequently, you may proceed comprehensively with the design process.
Upon completion of your design, you may go to the implementation phase comprehensively. There is limited scope for modification once a phase is deemed complete, as alterations can impact the software delivery timeline, expenses, and quality.

What is the significance of that?
This implies that you are currently in the implementation phase and engaged in a specific type of execution. 
A change in requirements has now occurred.
At this point, it was exceedingly challenging to implement any modifications during the process.
Any alteration poses significant challenges for accommodation, as it will undoubtedly impact the delivery schedule. 
This may undoubtedly elevate the cost, although ultimately, you will not get a superior quality goods.
So in waterfall model, if you have all the requirements in place, which is very difficult nowadays It is not feasible to obtain all client requirements on the first day.

The waterfall methodology is highly effective for smaller projects if all requirements are predetermined. It was straightforward to manage, particularly when all criteria were predefined.

if it's a mid scale or a large scale project, a project wherein the requirements and things are changing rapidly, and where you are where you are dependent on client to get the get the valuable feedback, then you are going and making changes, then the waterfall methodology is not appropriate.