When you know what needs to be delivered and the customer agrees about the problem and proposed solution then you should make a detailed execution plan. The first step in this journey of making a detailed execution plan is to list the activities and define the activities which need to be done to produce those deliverables. Thus the purpose of this process is to know what work needs to be done by the team and partners. This process also does the listing of all project milestones and attributes of the activities.
Define Activities (Process Summary)
This table is based on PMBOK 6th Edition of PMI