A hypothesis or purpose provides the starting point for your project. It can explain what you will be doing and why. It should be a short description and is typically a single sentence although it can be longer. The hypothesis/purpose is written before you start your project. It does not matter if the results of your project refute this or if you change what is worked on during the project.
A hypothesis is normally used for a project where research or experimentation is involved. The hypothesis is your guess as to what your research will reveal or show. The abstract that is done when the project is done can summarize whether the hypothesis was correct or not.
A purpose is often presented for engineering projects where there is a plan to be followed to build something. The research plan will provide the details. The abstract will summarize whether the purpose was met.
An example of a hypothesis is:
Using worm castings to fertilize Scarlet Runner Beans will result in more flowers for butterflies.
An example of a purpose is:
Our robot is designed to win the Robocup competition.