This is the 869th most frequent Japanese word.
いわ
Rock; boulder.
In this sentence, 'いわ' (岩) contributes to the compound word 岩魚 (いわな), which means a type of fish.
岩魚は清流に生息する魚です。
Iwana are fish that live in clear streams.
Here, 'いわ' (岩) is used to refer to a large, natural rock.
この湖の近くには大きい岩があります。
There is a large rock near this lake.
Here, 'いわ' (岩) is used metaphorically to describe something strong or unyielding.