、。
I totally love tennis.
Literal
I [topic-、] tennis [subject-が] big-love [explanatory-なんだ].
大好き ('to really like, to love') is the emphatic form of 好き, with 大 ('big') as an intensifier. A common lexicalized compound. Still takes が for the liked object, like regular 好き. なんだ is emphatic explanation.