USCE (US clinical experience) means clinical time spent in a US healthcare setting, such as observerships, externships or hands-on rotations. Many residency programs expect or prefer IMGs to have some, and it is the main way IMGs earn US letters of recommendation.
What counts differs by program. Hands-on experience is generally valued more than purely observational experience, and recent experience is generally valued more than old experience. See Observership for the most common type, and check each program's stated requirements.