Phases are meant to break up really large goals into different sections so you have a better overview. They can also be used for organizing trackers and habits.
Examples
A goal for getting fit and healthy can be broken up into different phases where each phase refers to one subdivision of that goal.
For starting a Ph.D. one can create a pre-application phase, an application phase, and an after acceptance phase.
A goal for writing a paper can be split into the following phases: research, writing, revising, and publishing.
If you want to read 40 books a year you can create 4 phases, one for each quarter.
Phases don't need to be in linear order. Instead, they can be used for better organizing projects and tasks in a goal. You can use the first phase for things you're working on at the moment and when you complete an item you can move another one into that phase.
If you have large projects you might be better off adding subprojects to your goal instead of the whole project. That way you can divide them into different phases.