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You just need to understand what kind of suffix '-osity' is.

Literal

'-osity' [called-という] suffix [subject-が] what-kind-of thing [question-か][object-を] understand-[conditional-ば] good.

~ば良い 'you just need to, it's enough if' — the conditional + 良い sets a minimum goal. 接尾辞 'suffix.' どういうものか 'what kind of thing' is an embedded question. A study guidance sentence.