This is a principle for curriculum and lesson planning. It is also called Backward Design because you literally plan backwards from what you want the learning outcomes to be.
The first step is to decide what it is you want the student to know, understand, and be able to do for the long term. This generally coordinates with standards.
The next step is to develop at least a draft of your assessment tool. Make a test that has all the information you deemed important to learn from the previous step. You may decide later to give choices in assessment, such as allowing oral, written, or verbal presentations. You might decide to allow group work, projects, or authentic/real world application assessments (with an individual accountability element of a written reflection done independent from the group afterwards). However you decide to assess, the basic content of what you want to cover needs to be established by drafting your assessment criteria.
The final step is to plan how you will teach the information you decided was a priority to meet standards and focus on what students need to know and understand for life. Even though we often think of planning lessons coming first, we must analyze what we want as the learning outcome before we decide what and how to teach. You can see how additional information may be added for those students with deeper interests or abilities with the specific topics, but for those who struggle, prioritizing is important.