Planning is the activity where you decide your testing strategy (e.g.  which requirements you want to validate, how to validate them, use either manual or automated tests), the resource allocation, and the execution plan (whom and when will execute the tests). Depending on several factors, you may want to prioritize some of the tests, or give more focus on some requirements. 

Although you may plan tests in different ways, Xray v2 introduced the Test Plan issue specifically for this purpose.