The gradebook is linked to assessment activities in FLO. All the grades for each student in a topic can be found in the topic gradebook, or 'Grader report' in the block Administration > Grade administration. The grader report collects assessment items that have been graded from the various parts of FLO and allows you to view and change them, as well as sort them into categories and calculate totals in various ways. When you add an assessment item in a FLO topic, the gradebook automatically creates space for the grades it will produce and adds the grades themselves as they are generated, either by the system or by you.
The grades are initially displayed as the raw marks from the assessments themselves, so will depend on how you set those up (e.g. an essay out of 36 will appear as however many raw marks that student got, not as a percentage – although this can be changed later).
When you create gradeable activities in FLO (such as assignments, quizzes and peer assessments), grade columns are automatically created in the gradebook. For all other assessment items, you should manually create grade items.