Add an Attendance activity and add sessions for which you want to record attendance. Go into each session and record attendance for individual students or in bulk.

To allow students to record their own attendance in Moodle, there are a number of settings to be configured. It can be configured so that if students fail to register they are recorded by default as Absent. It is possible to restrict how long the "register" is available to students for each session and to specify a unique password and QR code for each session to allow for recording of attendance from genuine attendees.