"나 잠시만 뭐 사가지고 올께" => I'll go get[buy] something really quickly. - 나: I - the subject. The topic/subject marker 은/는, 이/가 may follow the subject but not always, as in here. - 잠시만: 잠시(a moment, a short while) + 만(only) => just for a while, briefly. - 뭐: contraction of 무엇 (what, something). The object marker 를 is omitted. - 사가지고: 사(buy) + 가지고(with something; carrying something) => "buy and ...". - 올게: 오다(come) + ~ㄹ게(sentence ending expressing an intention). "-ㄹ꼐" is a nonstandard spelling. Grammatically, you may think of it as 나는 잠시만 무엇을 사(서) 가지고 올게. But this is how it is said most of the time.

"let me go buy something really quick."

Wait a minute I go out to buy something