Whenever my father has even a little free time, he goes fishing.
Literal
Father [topic-は] free-time [even-さえ] if-exists fishing [purpose-に] goes [polite-ます].
~さえ~ば is a powerful conditional: 'if only X, then always Y.' Here 暇さえあれば means 'if there's even the slightest free time.' This expresses an irresistible habit — the minimal condition for fishing to happen.