。
The caged tiger whipped its tail vigorously.
Literal
Cage [locative-に] be-put-[state-られた] tiger [topic-は] vigorously tail [object-を] shook.
A descriptive past-tense sentence. 入れられた is the passive past of 入れる ('to put in') — the tiger was put in the cage. 盛んに ('vigorously, actively') is a formal adverb. 尾 (お) 'tail' is the formal word (しっぽ is colloquial). 振る ('to shake, swing, wave') — the same verb used for shaking hands, waving, and a cat wagging its tail.