Walaikum salaam wa rahmatullahi wa barakatuh,
In the most general sense a Shia is a Muslim who believes in the wilayah of Ali ibn Abi Talib, and the Imamate of his progeny as dictates by the Messenger of Allah and the Qur’an.
Wilayah is mentioned within the Qur’an (5:55) and the Sunnah carries the Imamate of his progeny. As opposed to earthly leaders Shia follow the divinely chosen leaders and pious examples.
If more detail is needed feel free to ask.
insha’Allah you, your family, and your associates are in the best of health, iman, and security.