にくい is written as 難い and it means “difficult”, so the above sentence means “Fish is difficult to eat.” the Opposite is やすい（易い）and it means easy.
If にくい or やすい is used with a verb stem, that means “it’s hard to …” or “it’s easy to ….” respectively.
Juices are easy to drink but maccha (powdered green tea) is hard to drink.
Roomaji is easy to read but kanji characters are hard to read.
This pen is easy to write with but that pen is hard to write with.
This music is easy to listen to but that music is hard to listen to.