A Holter Monitor is a portable EKG that monitors the electrical activity of a freely moving person's heart generally for one to two days, 24-hours a day. It is most often used when the doctor suspects an ischemia (not enough blood flow to the heart muscle).
It is a painless test; electrodes from the monitor are taped to the skin. Once the monitor is in place, you can go home and perform all of your normal activities (except showering). You will be asked to keep a diary of your activities and any symptoms you experience and when they occur.