An employee can be absent for multiple reasons. There are a few ways to add an absence.
The employee can add his/her absence through the mobile app > Absence
You can do it via de desktop version and go to Schedule > click the white arrow in top right where it says + Add shift > select Absence.
Another way to this is when you go to Employees > select an employee > go to Absence and you add a new absence using + Add absence.
For more information about absence click on this link.