。
She gave her son a light pat on the shoulder.
Literal
She [topic-は] son [genitive-の] shoulder [object-を] lightly tapped.
肩をたたく ('pat on the shoulder, tap on the shoulder') is a standard collocation. 軽く is the adverbial form of 軽い ('light'), used here for the gentle quality of the tap. The body-part-をたたく pattern works for various tap targets: 肩をたたく ('pat shoulder'), 頭をたたく ('pat / hit head'), 背中をたたく ('pat back'). A pat on the shoulder is a culturally lighter physical gesture in Japan than hugs or other contact, and often carries a comforting or congratulatory weight.