The planning phase of project management allows you as the main planner to create a list of products and materials you may need as well as create a schedule. Typically, you are up against a deadline in which the project needs to be finished.
The first step is to calculate all the steps that needs to be done and calculate how much time each step will take. You then create something called a critical path. The critical path represents the order in which the steps need to be taken and the shortest amount of time they can be done. It also determines which steps can be completed simultaneously and which steps need to be completed in order to start a new one.
For example, if your project was building a new home, you cannot have the step of painting walls come before the step of building the roof.
This image above represents a model of a critical path diagram. As you can see, each task is labeled along with the duration they take to complete. Moving left to right, we see that tasks A and B can start at the same time. However, we also see that task D needs task A to be completed in order for it to start.
The duration for the critical path is determined by finding all routes from A to finish that include all steps. Of the different paths, the longest path is labeled as the critical path. This is the shortest amount of time in which all steps can be completed in the correct order.
This link also provides more insight into the planning phase. Information such as planning efficiency and effectiveness is discussed.