。
I'm bad at swimming.
Literal
I [topic-は] swimming [subject-が] unskillful [copula-や] [explanatory-ねん].
A Kansai dialect sentence. やねん is the Kansai equivalent of standard なんだ — や replaces だ and ねん replaces んだ/のだ. 下手 (へた, 'unskillful, poor at') is a な-adjective. This dialect form is very common in the Osaka/Kansai region and instantly marks the speaker's regional identity.