The device that Eriksen has been fitted with is known as an ICD, or an implantable cardioverter-defibrillator. This device is implanted under the skin near the collarbone and connected to the heart via tiny wires.
In patients with known heart problems, the device is able to recognize an irregular or stopped heartbeat, and can deliver an electric shock to restart the heart, just like an external defibrillator would do. Some newer versions of the device also have the ability to double as a pacemaker, allowing it to not only shock a stopped heart, but allow it to deliver more restrained electric currents to regulate a slowed or irregular heartbeat.
Other professional footballers have received this device in the past. Fabrice Muamba, whose heart stopped for 78 minutes while playing for Bolton, was fitted with this device after making a miraculous recovery, although he never played professionally again.