In absentia means "in the absence of". In law, this term is commonly used in reference to people who are sentenced without being physically present in the courtroom. This can happen if a person flees the state, not wanting to risk being found guilty, or if he or she is absent for other reasons.
An example of in absentia being used in law is when Nazi leaders were charged with war crimes after World War II, and they then fled, hoping to avoid conviction. In these cases, the Nazi leaders were sentenced "in absentia". Not all criminals who are being tried for crimes simply show up to court and receive their sentences. Sometimes they flee if they are not being held in jail. In these cases, they can be hunted down by law enforcement to help ensure that they go to prison or receive another punishment.