。
My friends call me Ken.
Literal
Friends [topic-は] me-of matter [object-を] Ken [quotative-と] call.
僕 is the first-person pronoun used by males — slightly more casual than 私 but more formal than 俺. Here it appears with the 'softener' のこと ('the matter of [me]'), which is a common way to refer to oneself as the object of an action without feeling too direct. Very common with verbs of knowing, loving, calling, thinking about.