、。
Playing baseball is a kind of sport, and so is mountain climbing.
Literal
Baseball [obj-を] do-[nom-のは] one kind [gen-の] sport [copula-だし], mountain-climbing [also-も] one kind [gen-の] sport [copula-だ].
~し is a coordinating conjunction meaning 'and also, moreover' — it typically presents multiple parallel reasons or facts. 一種 ('one kind, a type') is a classifier noun meaning 'a kind of' — 一種のスポーツ 'a kind of sport.' The structure 'X も Y' is symmetric, emphasizing that both belong to the same category.