The arresting officer is the police officer who has arrested you. It might be that you were arrested at the scene of a crime, or later following an investigation. The police officer could be of any rank and they will be recorded as the person who arrested you.
Usually, but not always, they will be the person who interviews you. This might depend on the time of arrest and whether you are fit to be interviewed straight away. They are then likely to present their findings to the Custody Sergeant for them to decide how to proceed.