What is "preservation of error" and why does it matter?

Preservation of error means properly raising an objection or argument at the trial court level so that an appellate court is actually allowed to review it later. If a trial lawyer never objects to something or never puts an argument on the record, an appellate lawyer typically cannot raise it for the first time on appeal, regardless of how strong the argument would have been. It matters because it determines, well before any appeal is filed, which issues will even be available to argue if one is.