To be an absentee means you are not in attendance... you are not there... you missed an event... or simply, you are without.
The effect this has can vary, but essentially, you will not partake in, nor be impacted by any occurances 'at' an event. That can be good, and it can be bad;
The tendency is to use 'absentee' in the negative (missing class, missing a meeting, missing work, etc), but sometimes, being an absentee 'can' be a good thing!