It's a portable EKG that checks the electrical activity of your heart for 1 to 2 days, 24-hours a day. Your doctor may suggest it if he suspects you have an abnormal heart rhythm, you have palpitations, or don't have enough blood flow to your heart muscle.
Like the standard EKG, it's painless. The electrodes from the monitor are taped to your skin. Once they're in place, you can go home and do all of your normal activities except shower. Your doctor will ask you to keep a diary of what you did and any symptoms you notice.