ST = Sinus Tachycardia. It means the heart is working down the normal electrical system, but its going faster than normal. Often there is an underlying cause (and not a primary problem with the heart), but sometimes peoples hearts just go fast and we need to slow them down with medications like your toprol.
28YO female, healthy and an avid lifter. Referred to cardiology for heart palpitations and fast heart rate. Placed on 7 day heart monitor. Was told that I am in ST majority of the day. There were some skipped beats as well. Placed on toporol for management. Cardiologist said borderline concerned, due to how high my HR is. Said if it gets worse, things would change. These strips are from the Holter monitor report.