。
Falling rocks are dangerous for mountain climbers.
Literal
Falling-rocks [topic-は] mountain-climber [for-にとって] dangerous is.
~にとって ('for / from the perspective of') frames the subjective standard by which something is judged — here, 'dangerous [from the climber's perspective].' 登山者 ('mountain climber') is a Sino-Japanese compound: 登山 ('mountain climbing') + 者 ('person'). である is the formal written copula.