its a great idea you got a BP monitor at home. Blood pressure readings are dual, the one on top, usually the higher is the systolic, and the one below, lower is the diastolic. 58 simply means its your diastolic blood pressure. I think you continue to record your blood pressure. If the higher exceeds 140 and the lower 90, its a sign of hypertension
Also remind your doctor if he forgets to discuss your blood pressure level values with you.
Thanks and regards,