Former TPS here. Your primary role is to deter theft and shortage by being a visible security presence in the store. Expect a lot of computer training in your first few days going over what you can and cannot do as a TPS, including best practices for keeping you and your fellow TMs safe when situations escalate. You will also learn how to properly document incidents and how to work the cameras (that is the fun part).
In terms of what to expect, AP can go from the most boring to the most stressful job in the store in a matter of seconds. Depending on the amount of theft that occurs at your store, your AP team could be just you and your TL/ETL, so you may find yourself feeling isolated when the rest of your team is not there. You will have access to a lot of confidential information, so just be careful regarding what you share and to whom.