When you do something in absentia, you're not physically present. For example, when you graduate in absentia, it means that although you get a diploma, you don't actually attend the ceremony.
In absentia is usually a legal way to describe someone as being absent, especially during a courtroom trial that the defendant doesn't attend. A judge or lawyer might say, "He is being tried in absentia." More often, it's used as a criticism of a trial, since most legal systems give people the right to be present at their own trials. Simply enough, in absentia means "in his absence" in Latin.