An observership is a clinical placement in which you shadow US physicians and observe patient care without hands-on responsibility or direct patient contact. It is a common form of USCE for IMGs because it does not require a license or student status.
Observerships vary in length, structure and cost. Some programs value them less than hands-on experience, but a strong observership can produce a meaningful letter of recommendation. Visa requirements for observerships depend on your country and situation.